Im a caterer (basically same thing) so I believe I can shed some insight.Well in my line of work I usually cater to weddings so I have an added bonus there.
Sometimes while Im serving a table and Im sure this is the case for you as the girl(s) just keep watching your every move.I mean its normal for customers to pay attention to waiters but Ive seen it happen to me even when its not needed.
If they are with their family,then just dont try.Like for me I think it would be extremely awkward to get a number while they are in front of family.I mean you could small talk and ask what school they are from and such and say you have seen em around but thats about as far as you should go.
In one instance this table I was serving with a family had this girl just keep STARING AT ME constantly.I mean non-stop staring as I was delivering food and I could tell from corner of my eye.She was eye ****ing me a ridiculous amount.So as her parents and grandparents go up to dance and shes all alone
me:you look like your having fun (sarcasm)
her:yeah lots of fun
me:you know,if i wasnt wearing my uniform now,id probably ask you to dance haha but unfortunatly I cant
her
laughs alot)
Fluffed it from there and got her facebook last minute before she left.
Though what I did is what you can do aswell.As she was leaving her parents were saying the usual bye to other friends and I just waved my hand to signal for her to come to me.A very simple isolation like that then got the facebook easy.
If you are sure she likes you,just say "You know what I didnt get a chance to talk with you but I sure would like to.Gimme your number and ill get back to you soon" or just hand her your phone and say "listen,i dont wanna play no games.do what you gotta do" only if you are sure shes interested