If the groom is a member of the military and he wants to wear his uniform, then he should do so. However, if some of his attendants are in the military and some are civilians, all groomsmen should dress in civilian clothes for a unified effect.
Like the bridesmaids, the groomsmen are expected to pay the fees for their attire. After the wedding, the groom may ask his best man or one of the ushers to gather all the rented outfits for their return to the store.
Like the bride and bridesmaids, the groomsmen's outfits should echo the grooms own attire. Most grooms will choose to buy or rent a tuxedo, and the groomsmen should follow suit, renting tuxedos in the same style by the same manufacturer. Matching shoes may also be rented for a more uniform look. Of course, there should be some element of the groom's outfit that allows him to stand out—perhaps a different vest or cummerbund and tie, a fancier boutonniere or different cufflinks and studs.
Tip: If groomsmen are renting their tuxedos, make sure they try on every element—including cufflinks, studs, and shoes—before leaving the shop to make sure the outfit is complete and everything fits.