I may be imagining things. But I thought I used to be that if I hit the "reply" button under someones name in a thread it would insert their name and @ into my message. I could do this for several people I wanted to tag. I can't seem to do this anymore for multiple people. It works only works on the first.
Hm ... I've never used it that way so I can't be sure. If anyone else can confirm that this worked, please let me know! That does sound fairly convenient though.
In the meantime, when I want that sort of functionality I just manually type in their username (e.g. @John-Cox )