Generally the duties of a groomsman:
- Help best man plan the bachelor party
- Attend all wedding related festivities including rehearsal and rehearsal dinner
- Sometimes help with ushering guests before the ceremony
- walk with a bridesmaid down the aisle and stand next to groom during ceremony
- Depending, sometimes you’ll need to make a speech or toast.
- Make sure the dance floor isn’t empty
As for the tux, also depends on the couple. For us, we’re paying for the tuxes. Since it’s 5 months out, there may be a chance that you get to choose your tux which may mean you might have to pay for it (you should either be getting fitted as soon as this month or the next). If you’re also close to the bride, I’d honestly ask her if there’s a certain style and color she wants you to wear.
If not, ask the groom. Also ask the best man how you can assist with anything.