GitLab: document that one can @mention a group

wiki/src/contribute/working_together/GitLab.mdwn

@@ -344,6 +344,11 @@ ask your question in a comment there, mentioning them
 with their GitLab login name: `@nick`. GitLab will send them
 an email notification about it and add it to their To-Do list.
 
+If you want to raise attention of every single member of a team, mention it with
+the name of the corresponding [[!tails_gitlab explore/groups desc="group"]]:
+`@xyz-team`. GitLab will send an email notification about it to every member
+of this group, and add it to their To-Do list.
+
 ## Acting upon input requests
 
 It's important to provide requested information as quickly as you can,