The other thread about songs sung by someone other than the lead singer made me think of this. Some groups are set up with different members sharing lead singer duties. 

This could turn into a long list, but I'll start:

Beatles: Lennon, McCartney, Harrison. (Ringo had his songs but I wouldn't classify him as someone who is sharing lead duties.)

The Beach Boys: Mike Love, Carl Wilson, Brian Wilson.

Moody Blues: Justin Hayward, John Lodge, Mike Pinder, Ray Thomas.

Fleetwood Mack: Stevie Nicks, Lindsey Buckingham, Christine McVie.

Pink Floyd: David Gilmour, Roger Waters.

Styx: Tommy Shaw, Dennis DeYoung, James Young.