Restrict selection of available watchers in a project

[tlmueller]

We have different projects (ServiceDesk and Software Development Projects) organised in Jira.

In a specific scrum project we would like our clients to administer the watchers of a ticket. So we gave them the appropriate permission. Unfortunately,  every project member now can see all users existing in our Jira installation and assign them to a specific project ticket even though they might not be added as project members.

Is there any way to restrict the users available in the watchers selection so that only users can be selected who are member of the project?