The title asks the question, thanks.
When I try with my friend it just spits an error. Something like “the convoy leader could not give you permission to join the session.”
No, you cannot play FFA as a convoy to discourage any sort of attempt at making a “team” effort in an FFA event. You can however, queue for a match at the same time in an attempt to get into the same session as a friend, but it would take some luck to get into the same session this way.
In ranked it seems it is broken or something since it doesn’t say that you can’t do that in convoy, never could join in ranked. But in unranked you can join in a convoy.
yeah that makes sense I guess… 