How to add watchers to support ticket in the Jira Web Frontend
In the issue view panel click on "Workflow".
Click on "Add Participants".
Enter the email address you want to add to the ticket and confirm.
Please make sure to provide a valid, existing email. The email address will receive notifications regarding the specific ticket and will be allowed to comment on the issue via mail.
For data security reasons we cannot provide a User Picker here, as it cannot be restricted to show only a subset of the existing JIRA users.