Example: You've made a lobby with a room code. A couple of your friends join in. You'd like to play with a bigger group, but you could only get a few friends together.
My suggestion: Once all the friends have arrived, the host can optionally press a button to open a private lobby up into a public one, and then a bunch of strangers drop in from people queuing up into regular Loop mode.
At current, the only way to mix both strangers and friends in the same game is to press Enter Loop at the same time and cross your fingers, but that doesn't always drop you in the same game.