I'm sure someone has written this before, I searched for it, and found nothing. The thing is I just went on the MP and was put in a lobby but nobody was joining? Help. Can I fix it by going on SP, then MP?
Did you check the usual? If your NAT is Strict/Moderate it can restrict it to not find people for searches. Same with bad connections. Also, what game type were you looking for? Sometimes there isn't enough players in some of the various playlists (usually the less popular, Assault, Extraction comes to mind).
So, do the usual checklist:
NAT
Connection Speed
Player Population (Can't check physically currently, but take in to consideration, try a TIA if nothing else to make sure your in a playlist that has people)
Locale (On the Dashboard)
Let us know if you've done all that or had further issues.