What I would do here is have a lobby server that would do things like user account validation, and would send the player a list of servers that the player can then choose to connect to. This can be seamless to the player (choosing the lowest populated instance for example), or manual (letting the player choose which server to join). TNet can do all of this for you easily. Just keep in mind that actual load balancing logic is up to you as TNet doesn't have that.