In Mafia Wars, a player can only become so powerful by himself. The size of your Mafia directly influences the amount of damage you deal and take during fights. In this guide, you'll learn the benefits of having a bigger Mafia, how to manage your Mafia and how to grow it as quickly as possible to the 500 member maximum.
The Bigger, The Better
Aside from being required for higher level missions, Mafia members help with Player Vs. Player fights (Hitlist and Fights). Each one of your Mafia members uses 1 Weapon, 1 Armor and 1 Vehicle during each fight- so if a player with no Mafia attacks, he's just attacking with his own 1 Weapon, 1 Armor and 1 Vehicle, whereas a player with 500 Mafia members uses 501 Weapons, 501 Armors and 501 Vehicles (1 for each of your 500 members and 1 for yourself).
But They Don't Fight With Their Fists
As forementioned, each Mafia member attacks with 1 Weapon, 1 Armor and 1 Vehicle. If you have 500 Mafia, but just 1 Weapon, Armor and Vehicle- they are doing nothing for you. It is your responsibility as the Mafia Boss to equip each member with the best equipment you can afford.
Your Mafia members will automatically use the best equipment you have, and should you not have enough of the best, the remainder of your Mafia will use your 2nd best items. If you run out of the 2nd best, they will begin using your 3rd best, then 4th, and so on.
Example scenario: You have 500 Mafia, 50 Crowbars, 50 Pistols 100 Tommy Guns, 300 Chain Guns... The first 300 attackers will use the Chain Guns, the next 100 would use Tommy Guns, the next 50 would use Pistols and the last 50 would use the Crowbars. This same scenario can be applied to Armors and Vehicles.
Growing Your Mafia
When Mafia Wars first came out, anyone could add anyone else to their Mafia with the click of their mouse. Since this allowed people to grow from 0 to 500 way too quickly (I grew my Mafia in about an hour), the creators have changed the system to require all Mafia members to be friends of yours on MySpace before allowing you to invite them to join your Mafia.
Adding a Mafia member is now a 2 step process: first, you must add the player as your friend on MySpace or Facebook. After they accept your request, you must then go to your My Mafia tab in Mafia Wars and invite them to your Mafia. At that point, the next time the player logs into his/her Mafia Wars they'll be prompted to accept your invitation- once they accept you will both be added to each others Mafia.
Source: Mafia Wars Cheats and Strategy