It works through tree-to-tree hints. If you receive a hint from someone with mutual ancestors and want to ask them a question, you can send a message request. If they accept, you can start chatting. Vice versa if your tree generates a hint for someone who wants to chat with you.
Useful links
Tree-to-tree hints on Findmypast UK
Tree-to-tree hints on Findmypast United States & Canada
Tree-to-tree hints on Findmypast Australia & New Zealand
Tree-to-tree hints on Findmypast Ireland