I have created a plugin to mention user groups in Jira within the @User mention.
The plugin is called: User Mention Groups and has just been released in a new improved version (3.2.0) for Jira9.
With this plugin, each user/administrator can create their own mention groups and add them to an issue in the ticket details or in the comments using the @mention function. The group name is automatically replaced by the group members, as if all users were entered individually.
In addition, administrators can create global groups that can be used by all users in Jira.
Or you can give different users the authorization to create global groups.
If groups are only to be used via global group assignments (creation via tickets), the admin can deactivate the "Own groups" function for the users.
If you want to inform all users at the same time, you can also use Jira system groups.
By default, only administrators can use the system groups, but the option can also be activated for all users. Do you want to allow system groups but you only want to allow certain groups you can blacklist the system group and this groups are not able to select from other user than admins.
Furthermore you can add user to a mention blacklist. This user are not able to mention other user or groups.
Thx for your feedback or feature requests 
Best regards
Daniel
