Why Jira is not allowing me to assign or mention specific users which already have a Jira account?

[Janluca MARIS]

Hello, 

jira si not allowing me to assig tasks to some of the users that have already a Jira account, the same thing happens when I try to tag them. 

This is working for all the other users, it's not working only with few of them. 

Thank you very much for your help. 

[Laurie Sciutti]

Hi @Janluca MARIS ~ you must have the "Browse users and groups" global permission to tag users.