So, say I have a player toss a grenade into a group of minions. This group is at engaged range of each other. Do I still need to trigger blast to get extra damage on the group?

In the rules, it says outright that a GM has the authority to decide if Blast triggers automatically. Your example is a time when I would do so. Another example (which might be in the text?) is if the grenade is thrown into a small room.

So, say I have a player toss a grenade into a group of minions. This group is at engaged range of each other. Do I still need to trigger blast to get extra damage on the group?

Per RAW yes, unless there's some special situation like the minions are all in an especially confined space or something.

As a GM you could always say that it takes only 1 to trigger the blast effect because it is a small room, they are in an alcove, etc. But you could always explain away though that one of the NPC's pushed another one and he fell on the grenade thus it did not do much damage, or it rolled into a corner, or the dreaded semi-dud.

Just some ideas, I have done that in the past.