As my group and I use it now: Yes it is error prone. But if you could choose the group from a dropdown menu like the aspect-dropdown, tag could be automatically placed in the bottom of the post - like the right aspect could automatically be chosen.
I think it could be useful to take what is already there: tags (whichs create a stream only consisting of concerned posts) and aspect/public (targeting only concerned users), instead of reinventing a lot of stuff. I already use a combination to create a group, and it is working.
What I do see as a problem is having tags that are not used by someone outside the group. This would pollute the group stream with irrelevant posts. I have solved this by choosing a tag that I anticipate will not be used by others. I do not know if you could reserve a special kind of hashtag like the ## and ### i propose above to groups, maybe someone else with a better understanding of the code can enlighten us.