ActiveCollab supports multiple ways to @mention people.
Type @name to mention a certain person. If they're currently unavailable, the Availability record will appear next to their name. Hover to see if they're on vacation, taking a sick leave or a day off.
Note: They need to be already invited to a project.
Type @team-name to mention an entire team at once. The parenthesis will show how many team members are invited to the project, and the hover will list them.
@Task/Discussion/Note or @Here
Type @task/discussion/note or @here to mention all those already subscribed.
@Project or @Everyone
Type @project or @everyone to mention all those already invited to the project.
Type @leader to mention the project's leader.
Type @assignee to mention the task's assignee.