The Mob is an offensive tactic deployed in many team based games. Each game has its own way of doing it, as does Battlefield. The idea is that by having the entire team stay together in a group, any obstacle can be over come with use of brute force.
The Mob in Battlefield: Bad Company 2
In Bad Company 2, because players can spawn on their own squad members, The Mob tactic is very effective. An ideal team (of 12) is made up of 4 Medics, 3+ Assaults, 3 Engineers, and a single Recon. Each class has a role in The Mob.
The Medic's job in The Mob is to keep the rest of the mob in fighting shape. Minimizing deaths by healing and saving ticket resources by reviving gives the team a key offensive advantage. The Medic can also make heavy use of his LMG to suppress hostiles. After a push is made, medics should quickly move in to mop up and revive any fallen players. Health kits should be out and available for use at all times.
The Assault's job in The Mob is the key to victory. Assaults are expected to rush the enemy full force, using brute force and the element of surprise to basically Banzai a position. Medics will follow up the rear to revive fallen Assaults and keep the push moving. After a push is done, Assaults must supply ammo to the rest of the team and heal up. Residual defenders may still be in the area and the Assaults should be the first to deal with them. The Assault players will most likely die a lot, but Medics will ensure that the Assaults don't stay dead for very long.
The Engineer's job is similar to the Assaults. Engineers must also go in with the Assault players' charge to defeat hostiles. Although Engineers lack the fire-power of Assaults, additional guns make the push go smother. The Engineers' real job is as anti-armor. 3 Engineers can simultaneously fire on an enemy tank, crippling it quickly and easily. If there is friendly armor to be used, all of the Engineers should operate or escort it. The entire team should, in fact, escort the armor.
Armor should always be escorted by infantry. Engineers can either operate the tanks or standby to be ready to make inevitable repairs. Everyone else should use armor has hard cover when advancing. By putting the tank between you and the enemy, you focus the enemies' attention on the tank and off of you. Heavy tanks operators should always direct their attention at priority targets such as emplacements, buildings, and hostile armor. I advise using the Vehicle Warhead spec, as it can take out the entire side of a 2 story building with a single shell; you'd need at least 3 normal shells to do that. The turret gunner should keep on a swivel and engage soft targets at will.
As for helicopters, attack choppers such as the Apache or Havoc should do their best to bombard hostile forces into a corner. Cripple tanks, blow open buildings; fly steady and allow the gunner to target soft targets at longer ranges. Same can be done with the Hind. As for the Blackhawk, transport is the main role. 2 people, from two different squads, should pilot and gun. Squad mates from both squads spawn in the chopper and parachute into the hot zone. The gunner should do their best to harass soft targets and break open buildings. I recommend the Vehicle Smoke spec to deal with tracer darts.
Every squad gets a minimum of one medic but a maximum of two, and at least one assault if it can be helped. Engineers are advised, but not vital unless armor is popular.
Forming a Mob
Unfortunately, there is no way to create a mob, per say; mobs form themselves. There are things that you, the player, can do that can increase the likelihood of a mob being formed on your team. Mobs often form when the entire team focuses on a single objective. In rush, for example, if the entire team focuses on just one M-Com at a time, a mob may form. You can directly help make a mob, too. For starters, you're going to need followers. Your squad must be made up of people who all trust each other. You and you squad all must have faith in each other. Then, find another squad who's all together, and assimilate into them. You know have an 8-man posse. Roll together and fight together. Other players will notice this and see the "power in numbers" trope at work. The rest of the team may join in. At this point in the game, if progress is palpable, the mob will stay together.
General Tips for Success
Stick together, but don't get stuck together. If you get caught out or flanked, don't make it easy to get killed quickly. A medic should always be hiding someplace near by to cover the flanks if a counterattack is made. Assaults should make heavy use of their 40mm Grenade Launcher to blow open structures, injure enemies, and overall annoy and demoralize the enemy. Use the buddy system; never cross open ground alone. As dumb as that sounds, having a friend near by will give the enemy someone else to shoot at, and increase everyone's chances for survival. Do not mash RESPAWN; wait to get healed. You can be revived even when your counter hits zero. SPOT EVERYONE AND EVERYTHING. I shouldn't even have to say why.
The Mob in Battlefield 3
Just like in Bad Company 2, the Mob works great in Battlefield 3 for the same reasons. And, because the Assault and Medic are the same class, it may be even more effective. But, because on PC a team can be made up of 32 players, the ideal mob is still only 12 foot soldiers. If at all possible, the team should be split into 3 groups. Two ground units, and a vehicle unit. The vehicle unit is escorted by the foot soldiers and also take to the sky. A group of 12 is best made up of 6 assaults, 2 supports, 2 engineers, and 2 recons. Yes, recons are needed. One group should secure the flanks so the main force only need to worry about enemies coming from one direction.
The assault is the most important part of a mob. They make up the attacking force and are to shoot their way forward. Every squad will most likely have 3 assaults, but only one of those assaults need to bring med kits. The rest are recommended to bring grenade launchers. Smoke grenades are also welcome. Attacking an objective from all sides is best. Move cover to cover and keep it tight. Deploy smoke at downed team mates and try to revive them. Only rescue mates closest to you to avoid multiple players trying to revive one person.
Get out that machine gun and stick a bipod on it because suppression is the name of this game. Support players are to do exactly what their name implies and support the attacking force. Mortars and machine gun fire should constantly be peppering enemy positions. Mortars should scare enemies out of cover to be hit by the guns. The guns should scare enemies into staying behind cover so they'll be hit by mortars. Either way, they're screwed.
Always have ammo ready for your mates and stay spread out. Try to reload when someone else is firing so they'll be no shortage of suppression.
Engineers need to be watching out for vehicles. While the vehicle unit should have this down packed, they won't always be available at any given moment. Should enemy armor appear, engineers should fight back to draw attention off the main force and provide a distraction and even supplement friendly vehicle efforts.
When not engaging vehicles on foot, engineers can join the main assault force or ride in a vehicle. Should engineers have a Javelin and laser designation is being used, they can stick back with the supports and fire Javelins at everything.
There are two kinds of recons that are needed. The front-line recon brings the battle to the enemy by being a member of the assault force. An MAV to spot and track hostiles from the air, or a T-UGS they can throw down to keep track of enemies in the area. The front-line recon is the scout. A spawn becon is best placed outside so squadmates can parachute in.
The other is the sharp-shooter. The sharp-shooter should bring a Soflam and designate enemy vehicles when not counter-sniping or taking pot-shots at enemies.