I think it totally depends on the lead/project. I've been invited to do so on a project, and I felt I didn't have the time (or deep enough knowledge) to do so.

I think it totally depends on the lead/project. I've been invited to do so on a project, and I felt I didn't have the time (or deep enough knowledge) to do so. I would say all help is welcome in successful projects -- it has to be, for longevity