I sm seeing same issue with 360. Tried for 30 minutes to join my wife in a game. No go. Do not get option to invite friend, invite party or join friend when in a match - even when looking at open slots on scoreboard. These options are available when friend is in lobby between matches but, as Shamrock said, the game will give you an error that game is in a mode or between maps and unjoinable.
This has to be the most frustrating multiplayer setup I've come across in a decade which is crazy considering this is a multiplayer only game. I don't find garden ops as enjoyable as competitive mp but if I cannot play with friends other than by random chance I want a refund.