Ill take a stab at another explination.
a Group in Everquest 2 is anywhere between 2 and 6 players with a common interest or goal in mind. They will team up in a "group" to explore new content, farm items, kill named, complete quests, and so on.
A Raid refers to several groups of players, 2 to 4 Groups that gather together with a similar goal. It allows for a group of players to take down encounters that would normally be impossible for any single group of players to do.
It normally gives better loot, and higher experiance for quests.
Hope it helps.
By the way, you can see several different raid mobs from level 20+, so its not just end game players that can join in a raid.