I got this last night in Local Co-op. Only player 1 (host) got the achievement. The other players were not guests but 2 silver and 1 other gold account.
Originally I was going to try this with another player on Xbox Live but we didn't have enough players and you apparently can't call in a local co-op to an online game even if it's a gold account. So I attempted to do it with 4 local controllers and it worked.
I went through the tutorial until the first minion tower (the tower that when you attack it, colored squares come out and fill your minion summon counters). Then I attacked it with all four characters and summoned a couple minions each and got the achievement immediately.
If you continue through the tutorial from here and finish it you will also get the achievement