Java thread to poll Database

I have a Java Web application where when users login, their email is stored in 8 different user groups based on type of user.

Now I want to show email address of all users who are login in a particular group on UI(somewhat like when a user has done login, he can see who other users of same group have login) and this should be live status, so if some other user has logged out, it should be visible to all other users in that group. Is there some easy way.