Groomal

If weddings were organized around grooms, however, appropriate magazines and expos would surely emerge.[…]Groomal expos could be held in conjunction with boat shows.

Source: wiktionary

For the most part, however, the groom was physically absent or existed as a marginal figure within the pages of bridal magazines. Occasionally, he appeared humorously as the “Forgotten Man.” While he usually had his own section in magazines, it is clear that he was not expected to be the reader. Significantly, advertisers sought bridal business, not groomal business. In fact, men were a hindrance to consumption that had to be overcome.

Source: wiktionary

Modern historians have limited “dowry” to refer to the bridal-dos and “dower” to refer to the groomal-dos, but the latter usage has given rise to a good deal of ambiguity.

Source: wiktionary

Assuming your RIO is to appear as a member of the bridal or groomal party and fête your friends whilst wearing the official wedding frock of their choosing, you may have to resign yourself to looking a bit overstuffed in the photos, then “accidentally” spill some red wine on your duds during dinner and change into that roomy-yet-wedding-appropriate outfit you “totally forgot you had in your trunk!”

Source: wiktionary