This has been an issue for over 2 years with no fix.
I suggest you create a 50p server and send people through to private servers where you have your lobby, because this will probably not be fixed this year either.
https://devforum.roblox.com/t/server-is-full-on-a-game-with-1-player-servers/27289
https://devforum.roblox.com/t/singleplayer-server-is-full-bug/51707
https://devforum.roblox.com/t/player-automatically-follows-friends-into-1-player-servers/37777