I'm trying to create a ground attack mission in which you catch some AI planes taxiing to the runway and then taking off. Is there a way to do this? Can't figure it out. Thanks!

AI only taxi when landing. You could also spawn in AI getting ready to take off on the active part of the airfield.

Either way, you would have to time it so AI are just landing as you get there or they spawn and are starting the process of taking off.