Currently we don't have a build in feature to mention groups and we already have a feature request raised for that end:
My suggestion is for you to vote for that feature request and add your thoughts in there so we can increase it's relevance.
As a work around you could use below third party plugin add on, which is free, and seems to enable that end:
Hello Community 👋🏼 I’m Makisa, a product manager on Confluence Server and Data Center. Confluence Server & Data Center 7.0, our latest platform release, is now available and we wanted to shar...
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