According to Zynga ( from their official blog):
Hired Guns For Sale Free! You read it right! Hired Guns will soon appear on the Free Gift page and this is great news for Mafia Wars players who haven't acquired a mafia of 501. Each Free Gift will contain 2 Hired Guns and every player with a mafia of less than 501 will be able to accept 5 a day
This means that this way mafia will be increased a max of 10 members per day.
From free gifts page we may send hired guns to our mafia:
This Free Gift will be helpful for those needing to max mafia and for any reason having problem on joining new players to their mafia.
Anyways adding real helpful players is always a benefit, to get events (Challenge Missions, Special Events, Social Jobs, Social Operations,...) completed quicker !