Sub-Team tag / Group Tag

Hello KeyBase devs,

I'm new on your app and it looks great so far, thanks for your work!

I would like to share an idea that shouldn't be difficult and really "life changing":

Be able to tag a sub-team (in the general chat) even if we're not inside that specific sub-team.
An option in the general channel like "Allow sub-team tagging" maybe ? (This solution suppose that you already know the name of the sub-team, but it's not always true)

Or another way to do it would be to be able to apply a specific group on each user of the team.
* User_1 can be part of the marketing group but also games.
* User_2 can be part of dev and games.
* Admin of the team can add/remove tags on people.
* User can add existing tag to himself or create new tags
* No need to create a team or a channel to create a specific tag.
* No need to be in a specific sub-team to tag people by a group tag

So when you tag games you ping user_1 and user_2 for examples

I imagine that those tags would be part of the general team, and could be changed inside the team's settings.

Thank you very much, and keep up your good work!

No files, tests, or entry points are identified in the issue. First clarify which of the proposed group-tagging approaches is wanted and where team settings and general-chat mentions are implemented; done would require an agreed scope and acceptance criteria.

Written by the indexing model from the issue text.