Plan to sing whatever song you feel most comfortable singing. If there's any question about appropriateness (and not every group will care about genre...I saw people audition with everything from Twinkle Twinkle to musicals to pop to Rubber Ducky), make sure you have a back-up. If you really draw a blank, the group probably won't care if you turn to something standard like the National Anthem or Amazing Grace. Avoid anything too choral, avoid anything that doesn't showcase your voice (i.e. obviously no rap, preferably nothing too staccato, etc.). I'd also say avoid or at least be careful with the Boyz II Men suggestion because they tend to be very showy/solo-y and you'll want to show that you can blend (keep anything too solo-y and fancy to a minimum, but don't be afraid to do it a little bit). If the group has an obvious genre that it sticks to (such as R&B), then play to that. Otherwise, or if it's just kind of general "pop and whatever we feel like," do whatever makes you happy.
Most guys are baritones with a slight tenor/bass preference (most girls are mezzos with a slight soprano/alto preference), so your situation isn't uncommon. The only way it'll be a problem is if the group desperately needs one true bass and or one true tenor rather than just guys/members in general. The downside to a cappella auditions (well, not necessarily a downside...it can work for you in some cases) is that the outcome is so dependent on what spots need filling. But if you can really sing, you'll probably be just fine, regardless (leg up since you're a guy).
What school/groups? Good luck!
ETA: It is two words, but I believe that either one or two p's is acceptable. Don't quote me, but this was the result of ongoing battles in my own groups