Hello, I don't know why, but the users which are not in the JIRA Administrator group, can't @ mention anyone of the project. All the user have the same permissions also Browse Projects and all are in JIRA users as well.
Do they have the global permission for "browse users"?
Could you check Resolution mentioned in below link
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.