Issue with workgroup when update

I get authority to administre 360 users. One people left company . I edited the user profile and pass the account to another people.

I created one dashboard and published it, and then generated workgroup to make them access the dashboard. Now when I update dashboard, the old guy email address still showing in "Specific individual group" field of "ACCESS" page, I have to manually delete old guy email and enter new people email address, and then click update, please advise how to get rid of that