People are considered participant of a MR if username is present on a commit title.

Commits title could have a string formatted as @something that is causing that @something is added as a participant.

From chat:

@dblessing: @stanhu @asumaran In this case they became a participant not because of the commit itself, but because the name was repeated in the system note, which is processed like a regular comment. Users mentioned in commits are participants of that commit, and get notifications for comments on that specific commit, but they should not be participants of the MR. So I think it’s a bug. Can you create an issue Alfredo? It’s kind of non-trivial to fix, since system notes are processed like regular comments that of course should have mentions added to participants.

Steps to reproduce: On a existing MR

• Given usernameis a existing username.
• Push a new commit with something like @username in the title.
• Go to the MR page on gitlab.com.
• You will see a new note on the MR timeline with a link to @username and also @username icon is added to the participants section at the sidebar.