The connotation of sheep is pretty negative if used towards a group of more than 2 people.

For example, watching people all push and shove to get into one door at a mall or school, while some of the smart ones actually take the door next to it. Or those who vote for somebody only because "everybody's doing it." Or those people who watch MTV just because it's the popular thing to do.

Basically I'd define it as "somebody who did things without thinking about what they were doing." Popular among geeks

Oh, and it makes a good softlink, as I've been the victim of.