The grey or ghosted staff avatars on the dispatch board means that the staff member has 'clocked off' on the iPhone or iPad app - generally they should use this if they are on leave, or has finished work for the day so the office knows not to allocate any jobs to them.
As soon as they check into their first job (or clock in again) on the app, their corresponding staff icon will return to full color. So it's just a quick way to let you know they've clocked off - but you can still send them messages, allocate jobs, etc as per usual.
