There are three ways to control group chat membership:
Adding all users. This method adds every member of a domain to a given group chat and continues to add new users to the chat as they are added to the domain. By default, Trillian Server creates a group chat named "General" that does this for you as a way to provide an all-hands group chat, but you can configure other chats in this way as well.
Adding users by group. This method allows you to bind one or more groups to a group chat, such that as members join groups they are also automatically added to the group chats bound to them. You can use this to have your entire IT staff join an IT-specific group chat, for example.
Manual control. This method lets you add and remove members to and from a group chat as you choose, and also unlocks settings that allow users to join group chats on their own using various discovery features within Trillian clients.
In addition, all group chat members can receive two optional flags:
Op. Operators are per-chat administrators - they can change settings and assign operator status to other members at any time.
Voice. Voiced members can speak in group chats with the "only approved members may talk" setting enabled.