There seems to be a small error in Activity.
I posted a message and wanted to indicate that I was with one of the other members. When I entered the name of that person, using the appropriate button, there was no suggestion of usernames and the entered name ended up without a link to the profile.
When I enter the text @username in the message area, it is transformed into a link to the profile. So the @mention works fine.
Regards,
Peter