I find this limiting as well. I typically make a group for a project and do not use their old Project places. I use Producteev which works in a group. I only wish they would allow groups to be categories(coming soon) as projects and given the ability to be expired. Say like they could give the option of having the group be time bound for things like event groups and projects and perpetual for social groups and working teams.
For what you want I would suggest putting the people into a group and not using Projects, then use Producteev to administer tasks.
I find very little use of their Project places, probably why they bought Producteev.