Grownups rhymes with:
cleanups, lineups, runups, backups, buildups, buttercups, checkups, cleanups, coverups, eggcups, hangups, holdups, hookups, lineups, linkups, markups, pickups, push-ups, pushups, runups, setups, shakeups, startups
Grownups sounds like:
greenback, greenback's, greenbacks, greenbush, greenpeace, greenpeace's