A Group Summoner is special type of summoner. The key difference between a standard necro and a group necro is the curse "Lower Resistance." In a group necro we try to max that skill as much as is reasonable.
Here will max raise skeleton and skeleton mastery. We only put a single point into Skeleton Mage, Revive, etc. Because this necro is part of a group we don't want a zillion minions running around. A solo necro needs an army. A group necro needs a tight squad. The group summoner will typically run 5 skelly and 1-5 revives. Sometimes less, almost never more. There are rare occasions when a full army might be called for. But those occasions should be obvious when encountered. And don't forget the necro can always ask the group if it's time to raise an army.
The skills selected on this screen are Bone Armor and Corpse Explosion. Both useful skills. And with +skill points from equipment can be significant.
The group necro put one point into every curse simply because there exists rare occasions when each curse will come in handy.
But for the most part the necro will use one of three curses: Decrepify, Amplify Damage or Lower Resist.
Decrepify will slow down an advancing army, it will cause them to do less damage to one's team, and it will cause them to take more damage. Thus, creatures will attack less often, do less damage, and get hurt more. Not bad.
Amplify Damage will double the amount of physical damage a cursed monster will take. In addition, Amplify Damage can remove Physical Immunity (on occasion).
Lower Resist will lower the resistance of monsters. Thus many immunes will become "NOT IMMUNE." This is a major boon to magic users in the group such as the cold sorc and the traps assassin.
Add to this the stacked effect of a pally running Conviction and monsters will be almost naked to the group. I say almost because some monsters begin with a resistance of way more than 100%. So lowering them a bunch means they are almost vulnerable.
You will notice that in the screen to the right we have only 1 point assigned to Lower Resist. Here is the thing. At level 16, Lower Resist is pretty much maxed out at -61%. Adding additional points does not lower the resistance much at all. In fact at level 10, Lower Resist is already -55%. Each additional point until 16 will only lower by 1 percentage point.
Whether you shoot for a lower resist of -55% or -61%, it is still very wise to see how many pluses you get from equipment before you assign skill points to Lower Resist. So I'd recommend just putting one point in until you know your equipment.