To lock a room, use the three dots on the conversation window. There you can choose to lock or unlock the room.
Your team will be able to see that the room is locked, and if they click on the conversation they will not be able to join.
If people want to join, they must send a quick message to one of the members in the room (hover over that person's name to send them a chat message).
A room remains locked until everyone in the conversation leaves that room.
To add someone into a room once it's locked, hover over their name and click "Add.”