Oh, you mean free for all ...... you can still join in progress.
You can send him an invite or he can just watch until you are joinable and select join in progress.
It might take some waiting, as the lobbies fill up pretty quick. The most likely time for him to be able to join is when your match is almost over or right after it ends. ..... sometimes even once he map is selected you'll get some openeings.
Cahnces are, once you start playing a match, he'll just have to sit back and wait trough most of it.