Server selection uses Steam matchmaking. From the in-game menus, players choose Solo, Host, or join an existing server.
Cross-play caveat
Motor Town's FAQ explicitly notes the game "uses Steam's matchmaking system, which does not support cross-play between Demo and Full versions" — make sure everyone's on the same edition.
Company vehicles
Dedicated-server admins can configure whether players are allowed to bring company vehicles into the session. If the server has it disabled, your company stays in its home garage.