When you create a site you associate a site with three security groups (Visitors, Members, and Owners). Sometimes you want to change these default groups. For example, you want the requests for a site to go to a different group than the original Owners group. This can be a bit tricky but it can be done easily if you know the right page to go to (it is not under site settings where it should be). You can just append _layouts/15/permsetup.aspx to the end of your site to get to the page where you assign these groups.