Perhaps the ur-example of music is Jeff Ament and Stone Gossard of Pearl Jam. They've been in at least four bands together and have always played this role.
The three guys from Aerosmith that aren't Steven Tyler or Joe Perry. They're even jokingly called the Less Interesting Three among the band and fanbase.
An interesting case: Steely Dan, in the early days, had as members Michael McDonald and Jeff "Skunk" Baxter, who went on to become known for being in The Doobie Brothers. The two central members, Donald Fagen and Walter Becker, who wrote pretty much everything, and ended up being the only members, became the Face of the Band, and only known for Steely Dan. People actually forget sometimes McDonald and Baxter were in it, too.
Far more egregious is Denny Dias who was the band leader before Becker and Fagen joined.
Megadeth is those two guys Dave Mustaine and Dave Ellefson, plus whoever else Mustaine has hired. There were a few years where Dave Ellefson wasn't in the band, but he's back again, so it counts.
Rush is an interesting inversion: It's Neil Peart, Geddy Lee, and the blonde guy that plays the guitar. Poor, poor Alex. Of note is that if this were any other band Alex Lifeson would be the standout among Those Other Guys; Alex is very talented in his own right, but the poor guy is overshadowed by Geddy's vocals and front-row bass and Neil's drumming and lyrics.
To all but diehard fans of Pink Floyd, the classic lineup is Roger Waters, David Gilmour...and the drummer and keyboardist (Nick Mason and Richard Wright, respectively). During the band's early psychedelic period, Pink Floyd was Syd Barrett and those three other guys (Mason and Wright again, along with Waters, who at this point was just the bassist), and in the much later "dehydrated" (no Waters) lineup, Floyd was Gilmour...and those two guys playing drums and keyboard. Yeah, no matter who is the face of Pink Floyd, Mason and Wright always end up as Those Two Guys.