Control which administrators WPMU DEV Dashboard plugin

Control which administrators (manage_options enabled) can access/see the WPMU DEV Dashboard plugin and announcements. Note: ONLY these users will see announcements.

I have gone to dashboard -> manage -> permissions

I tried to add the admin (designer)for this site but when I add the full name the ADD button stays greyed out? Can you help.

I require this designer to have permissions only for this site.