after a bit of testing I have found out that I'm only able to use the tags I create my self.
Using a seperate test account I have two Teams sessions, and If I add a tag to a user when I'm logged on as myself my test account, who is also a member of the Team, is not able to use the tag.

I'll try to sell the "Open Team" solution. Maybe considering switching to Yammer for this exact scenario.
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