When playing in the Plaza, Condo, and games, accepting an invite to someone’s server yields no results. The only way you can accept an invite or join someone else is to first disconnect from your current server and reach the main menu.
If this was intentional, it feels like a bug. Having a little error screen appear mentioning that we need to disconnect first would be good enough. Otherwise, it can be a little aggravating after accepting an invite 3+ times and nothing happens.