Different bikes have different gear ranges based on the number of discs they have in the front & back, and the number of teeth on each disc. The lowest gear on your friend's bikes may in fact be lower than the lowest gear on yours , and if so it will be easier for them to spin in a lower gear than it is for you in a higher gear.

But, +1 to what Biciclista said...keep riding the hills in the closest-to-comfortable gear you can find; gradually as you get stronger you can try longer and/or steeper hills. This will serve as a good form of interval training and you may find that in the end it makes you climb more strongly than your friends who are having an easier time right now. That happens for me each year...my bike is geared higher than my husband's (I have a compact double, he has a triple). At the start of each season, he does better on the hills than I do because he can gear down while I'm tiring myself out. By the end of the season I usually torch him on the climbs because he's used to shifting down, while I've gotten strong enough to maintain cadence in a higher gear.