It is a good idea to note which end of pitroad you exit the garage when you join the server. That is where the majority of joiners will blast out of the garage right into you when you are pitting.
Example, Aston National, most join closest to pit exit, so make your pitstop at the earlier pits. Westhill is opposite where new joiners will come from near pit entrance, so pit towards the exit. It is different for every track.
Many times people are blasting on the throttle just as soon as their car joins in the garage, thus they blast right into you if you happen to be pitting in front of that garage area.