Unity 5 Multiplayer - Changing When Players Can Connect?

Hey everyone, I am trying to make a game which is mostly single player except for some areas/dungeons that allow co-op. Through Unity 5's UNET, I was wondering how I could make it so that other players cannot join the host/server until a certain point in the game.

For example, I have set up the network manager and the max connections is set to 1. But, this means that 1 client can join at any time. I want to make it so that client connection is blocked until the host/server specifically allows it to. (it doesn't let you set max connections to 0)

Is this possible at all?