The only hang activity really is making a play for the yard and extorting money from other prisoners to use objects in their territory. If a gang gets evicted, or a leader punished then gangs will see this as a moment of opportunity and will attack rival gang members their vicinity (happens in the shower the most for me).
Once a gang reaches 10 members a leader will arrive or be the "chosen one". Once the gang has a leader, a lieutenant will be promoted from a regular member for every 10 members in the gang. My largest gang at one time was somewhere around 35 members and they had a leader plus 3 lieutenants.
The gang leader is the only one who can recruit new members. He'll ignore regime schedules and walk straight to another prisoner to recruit. I've seen them both accepted and denied from other inmates. It is best to isolate the gang leader as soon as possible into another security sector. Don't put him in permanent lockup or solitary, or you will make the rest of the gang riot! If you don't want the leader to recruit then get him out of gen pop so he can't recruit the entire prison!
EDIT: Gang leaders will not be involved in making plays for territories which is awesome because then they won't get punished and disturb the peace. They may be in the yard while a play is being made but their presence won't have any effect.
On a side note; I thought the common rooms were also supposed to be a potential territory for gangs but I haven't seen that yet. Hopefully that changes. I'd also like to see gang members attempt to monopolize the work force by claiming a room such as laundry to extort money from prisoners by allowing non gang members to work out of that room.
