Groups, permissions and multiple SharePoint sites.

I am sure this is a common thing but I just don't know how to make this work.

I have 9 SharePoint's that the same group of people need to access. The group are managed on a SharePoint list right now and is regularly updated as people join and leave.

Currently I have to add and remove people from all 9 SharePoint's as the group changes.

What is the best way to create some sort of group that I add and remove users from that automatically updates all SharePoint sites?