verb. Hootchie-kootchie-ing and so forth, I reckon.
Real citation: "And tell him that while he'll have a blast with you two, he shouldn't pass on a date with a potential boyfriend—but so long as you three are friends-with-benefiting-it-up, you would like to be informed about any other sexual contacts he might have."
(Aug. 20, 2009, Dan Savage, The Stranger, http://www.thestranger.com/seattle/SavageLove?oid=2069897)
Made-up citation: "Humans aren't the only species renowned for friends-with-benefiting-it-up. Some of those hump-happy hounds in the park aren't even dating!"