Ingame groups.
An admin enables it with /enablegroups. They can disable it with /disablegroups.
If you have not already, you can type /startgroups NameGoesHere. This would make a group with your name. If you don't want to make your own, type /listgroups. It lists all the groups. Type /joingroup NameGoesHere and a request will be sent to the creator of the group. If they say /accept you are in. If they type /decline you are not in. /togglegroupdamage (admin command) will prevent your groupmembers from damaging eachother. This would be handy for Fortwars, Gang RP etc. There is also a /leavegroup command that notifies the other members of the group.