Basically, after you hit level 10, you can choose to sidekick a player of lesser *levelage* than you. For instance, say you are a level 20, and your friend is a wee lil 5. Normally, you two couldn't group. Well, if you sidekick him, he instantly becomes buffed up to have the offensive and defensive might of a level 19 (I'm pretty sure it's one level below the sidekicker). The sidekick still gains experience as if he were level 5 (No powerleveling) but it allows for people with large level gaps to still play together. It's an ingenious lil feature, and I'd love to see other games adopt it.
One note, I've never sidekicked before. This is just what I have seen from other forums, and what I've been told from other players, so if anyone sees something wrong, please feel free to correct me :) But I'm almost positive this is how it works.