PlanetsCentral

You explicitly underlined the "start from beginning", I guess I took that the wrong way.

By that I meant that the game should be opened for replacement players to join and they get to start with turn 1 (and not turn 2 or 5 or whenever someone decides to join).

Another solution could be to prevent people from joining if they don't have a confirmed-working email, but that would break the "oh, I want to play - click-signup-join" flow that I'd like not to lose...

You could still have the simple userflow and tentatively sign up the player instantly, but if they don't click the email confirmation within 24h then they're automatically unjoined.

I could take a slot if nobody objects; I don't have a current game.

I don't think anybody would ever object to you joining a game, come join the fun!