Solo is totally individual from multiplayer.

You can open your game up to others but need to start the game up that way.

Crypts are just areas you explore.

Multiplayer is max 10 people unless modded. Servers are all hosted by players. Almost all are locked due to people going in and doing harm. So to join others you need to find a group and ask for password.

So solo you don't need to worry about randos popping in or anything.