Yes, the lobby server can be your login server. You can save and load data on the lobby server, such as your player profiles. Authentication logic is something you'll need to add yourself, however.
Lobby server is just that -- a lobby (TcpLobbyClient for example). It doesn't have any game servers, and is completely separate from the TNManager.Connect logic you use to connect to an actual game server. You can be connected to both at the same time.
Lobby server is used to retrieve a list of game servers that you can then connect to, which is exactly what you need.
You can broadcast messages to everyone on the same game server, but you can't broadcast messages to everyone on the lobby server, as the lobby server has no concept of "players". It's just a server list.