yes, there are two ways to go about it. (you can join them in fighters or ground vehicles as well)
Probably the easiest way is to use the .join command. You would simply type .join (persons name) i.e. .join wmlute and once they accpet it, you are in the plane with them. You can man the guns on the bomer or ground vehicle, or simply fly along in a fighter.
BUT ALWAYS ASK before you send a .join, as it pops up a box on their screen that blocks their view, and in the middle of a fight that can be a lethal distraction.
The OTHER way is to go into the HANGER while you are in the tower, and in the middle left under the plane list, is a button that says JOIN. Click it. It will then bring up a list of pilots, find the one you want to join, highlight their name, and click ok.
Like the 1st example, this pops open a request to join box on their screen. ALWAYS ASK before you try to join someone.