thing is, i like who i go out with.....and i typically just hang out anyway, so i wouldn't be really thrown off by them inviting a friend....I can have the accumulative wealth of the top 3 folks on Forbes richest people list and if I didn't invite you on this outing I ain't paying for shit.
Start doing that, then they expect you to pay. Always
i met plenty of my wife's friends that way when we first started dating.....and if i had it i offered... if i didn't, i didn't..... but they never assumed.....and everyone got along fine....