In Activities you can assign a to-do to a single named individual, or 'anyone' - if assigned to 'anyone', then any member of the activity can mark it as complete.

 

What I think might be quite cool was if you were able to assign a to-do to multiple named individuals, or to 'everyone' (i.e. all members of the activity) - in both cases, the to-do should not be marked as complete until all assignees have marked it as such. This would be helpful in cases where multiple people need to perform the same action, like commenting on/reviewing a document, or booking themselves onto an event, etc.