Can't message groups that you cannot mention

[tgxworld (Alan Tan)]

Fixed in

https://github.com/discourse/discourse/commit/91c5f928c22289ffaccf349543ec1ff5694274c4
https://github.com/discourse/discourse/commit/8140e546754f787356f7d946de334959c89a4732

Thanks for reporting @pfaffman

cc @nbianca