Out-of-the-box nobody can send messages to groups or trigger mention notifications.
To change this, you can go to the Manage tab of a group’s page and select the “Interaction” section from the left navigation sidebar. Find the settings labeled “Who can @mention this group?” and “Who can message this group?”
Additionally you may adjust the group’s visibility settings.
When users perform “autocomplete” using the @ key, all visible groups will show up. This means that out-of-the-box you can talk about groups like @trust_level_0 and so on, Discourse will perform automatic linking.
Enabling @mentions will notify all of the group’s members whenever it is mentioned. When a user attempts to mention a group, they will see a message in their composer that says:
By mentioning @group, you are about to notify 2 people – are you sure?"
As a safeguard a site setting was added -
max users notified per group mention - which is defaulted to 100. Even if you enable mentions on a group with 101 users, no notifications at all will be generated when they are mentioned.
You can use a group name as target for a message (provided the group allows messaging)
This correctly links to the group from the message page:
All group mentions are tracked in the database, this allows us to display them in the group page under the Activity > Mentions section of the group’s page.
For example @trust_level_0 will make this post show up in the mentions list for the trust_level_0 group