By default, all users defined in the Known hosts table are part of the #Team room. New custom rooms can be created by entering a text in the drop-down below and pressing ENTER, but all users will be part of it:
A new room with custom users can be created by selecting the users from the left grid and pressing the New button. In this case, the room name cannot be configured.
To access the conversation history of a room with custom users you need to recreate again that room by selecting the same users.