Groups are important when it comes to Hide/unhide a group of object in a matter of seconds. Or if you want to re-locate/re-select a bunch of actors you assigned the other day without having to go on and re-select each one of them ETC ETC ETC. Such a pain right?
How to use groups
Open the generic browser, by default, you have Generic, Actor classes, then Groups.
On the left you can see the groups, with check box next to them to turn them visible on/off
As you probably noticed, almost every actors (Lights, Mesh, Player Starts and all that) have the same properties. "Object" is one of them, each actor has a tag, and a Group name. "None" is the default group that is always created, if you don't want your object(s) to be in a group anymore, simply type none or delete the whole space.
You can also have more groups for a single actor. To do that, just separate them with a comas. EX: Blockers, group2
Create a group by adding a name in the red field shown in the picture above and press "ENTER"
You should always create a group by adding a name in the Group field as is in the picture. Once you create a group, you will not see it pop up in the group browser. You will need to refresh.
You should now be able to turn them visible on/off. And select the actors of the group you just named.
You can also add actor(s) to your current existing group. But to be really honnest, the best way of dealing with all this is to add them manually in the properties as I mentioned earlier. If you don't like it that way, you can always EDIT -> ADD SELECTED ACTORS TO GROUP