This would be a great feature. Whenever I'm grinding stranger missions while my friends are offline, I'll create a temporary public posse and hope some random hops on who knows the basics. I welcome new players, but once I start explaining the weird "wait the clock out and hunt/gather for more $ and XP" they usually bail or LEEEROY JENKINS the mission.
I joined the Team Up community on here too, but not sure how to approach folks when joining their session or inviting them to mine. It needs some secret (free) emote when you approach people, imo.