The passers should have a tray with the item and whatever goes with it: a large enough dish of the sauce to dip into, for example -- and they should not let people double-dip. It's better to NOT have items that leave the guest holding something inedible, like the skewer, but since you're offering kebabs, the tray should also have a receptacle for "empties," preferably with a skewer already in it so the guests get the idea and don't leave sticks all over the house (I've seen that done because they didn't know how to get rid of them

). If the "empty" is gross, like a bone, use an opaque receptacle. The passers should also carry a stack of napkins, and should be able to hand one to a guest (preferable) or at least offer the stack for the guest to take one.
So it's: tray with food, condiment, and waste receptacle in one hand, napkins in the other.
Anyway, that's how I did it when I was catering. Others may have other ideas.