Trying to think of bands that are mostly male and have non-lead-singer female members.
Perfect example: Smashing Pumpkins with D’arcy Wretzky on bass.
Not a good example: No Doubt, Hole, Florence + The Machine
Other good examples:
Belle & Sebastian (even tho the girls sing sometimes, but not much)
Skankin’ Pickle
Lenny Kravitz…if we consider him a band, and his drummer Cindy Blackman
The Cramps, alas, stopped performing after Lux Interior died. Throughout the history of the band–& many personnel changes–he sang lead & his wife, Poison Ivy, played guitar…
Arcade Fire have a couple female members, occasional lead vocalist and multi-instrumentalist Régine Chassagne and violinist Sarah Neufeld.
Others that spring to mind, the White Stripes, The Decemberists, The Dandy Warhols, Silversun Pickups and Faithless. Female instrumentalists are definitely under-represented in rock music.
I was just about to say, I wasn’t sure if the Cramps counted as “mostly male”…I guess it depends on the specific lineup.
The early-mid '70s incarnations of the Mothers with Ruth Underwood. There were also the short-lived Zappa band lineups with Norma Bell and Bianca, both of whom contributed some lead vocals but weren’t the main singers.
Despite her singing lead on a fair number of songs, I think Christine McVie qualifies. She was always one of two or more singers and made an important contribution instrumentally as Fleetwood Mac transitioned from a blues band to a pop band.