The system I’m building is compatible with this, with a slightly different perspective.
Take Diablo 3 for example, which has a similar “join a friend” feature.
You can start a game by yourself. Other players can be invited, friends can join. It’s all “seamless” in the way you describe. But… In actuality, there is a multiplayer server involved from the very beginning. It never switches from a single player game to a multiplayer game… It was a multiplayer game to begin with.
There are many reasons to use this approach, especially now that creating and destroying dedicated servers is so cheap and easy.
Does that make sense?