Today I want to talk a bit about team composition. A more table top friendly term might be "Party Composition" but I'm specifically talking about in combat - however mechanical your game of choice makes combat - and how your players act as a team and what that means. Mostly this is about a few things to look for to know when you can ramp up the difficulty, and when you want to tone it way down.
A Good Comp
I don't want to talk about what your game should have in it. The "holy trinity" is basically a tank, a healer, and damage but every game does that differently. Also, I disagree that there needs to be certain classes or roles for a game to work. Yes, in some games there are optimal compositions, but that is so system specific I don't want to talk about it. No, when I talk about team comp I mean the synergy between the characters.
A Bunch of Individuals
This is the most basic form of composition and to be honest most groups do this - especially in the early sessions. At this stage the PCs are a team in name only. There is little to no teamwork outside of who the healer heals, and who is tanking the damage - and that is assuming you even have that. Each PC is more or less an island and approaching their piece of the fight themselves.
This type of group will engage multiple enemies as individuals even when that's not the best way to do it. There will be little to no team work aside from the occasional opportunistic moment - especially by rogues. You will also likely see wasted attacks (high damage attacks put into taking out weak enemies) or AOE attacks that also hurt party members, or can otherwise put allies or objectives in danger.
You want to be careful when making challenges for this group. The old addage "United we stand, divided we fall" is appropriate here and maybe a combat showing that there's more to combat than just swinging at one enemy is appropriate. However, if you don't want to wipe the group, or are teaching the group how to play, keep to simpler encounters or ones with clear, obvious high priority threats that need to be piled onto.
A Seasoned Group
You'll get this out of a lot of experienced groups. People are still interested in their personal power fantasy, and maybe the players and PCs haven't gelled, but everyone knows how to work as a group. You'll start seeing clear tanking and an understanding not only of combat mechanics - at least partial - but how touse them. Clear threats will be addressed and tanks. Opportunistic attacks, and low use but high damage/utility abilities will be used with an eye towards overall gain for the group instead.
However, in panic situations you'll see some of the cracks. You'll also see cracks in instances of misplays or less than ideal strategic choices. These are fairly rare though, since everyone is seasoned, but they're still there.
With these groups you can start to open up the monster list a bit. You can even skirt bringing in challenges above what the PCs should be able to handle. Teamwork makes up for a lot. Still, be careful.
This is a group that lives or dies by their synergy. This goes for all walks from how they engage, how they retreat, and how they assess threats. There's a reason a cited a specific game here, and that's because I've seen th is style of play most often in Shadowrun. Shadowrun is a high lethality game, with a lot of crunch in the mechanics (though some of it is faux crunch) and if you mess up, or don't play your role optimally, your group will get wiped on some jobs. Because of this the synergy you get in veteran Shadowrun teams is kind of crazy, but also awesome to work with.
Coordination of attacks, covering for holes in defense, and knowing when to move in and when to move back are second nature. This kind of group you can go all out with because provided you don't one shot them, they'll likely know when to back out. Just don't be surprised when they take out your super deadly enemy without the villain even getting a chance to see them before he or she ends up in his own lava moat.
I've seen this twice, and both times from the inside. At this point your group has gelled so well they will do things without communicating. Your PCs will arrive in a town, go in 6 different directions, and when they come back they'll each have gathered information and done specific things to make a grand thing in support of the party you weren't expecting. Actions and combat will get taken down with the same coordination of the veteran shadowrunners, but it'll happen even without preparation at times.
One thing to note with the Hivemind, and with the Veteran Shadowrunners, is that a new PC can throw everything off. There's a reason the new guy is the FNG, and part of that is because they fuck up synergy. If someone dies, if someone joins, you can't expect the group to work with the same coordination. If you do, the person who is going to get caught out is the FNG too, because they're the one without the defined role, but then you have one player chain making new characters.