Redmine

Hello, I have group with a lot of users in it. And this group is participating in almost all projects in redmine. Simple group deletion is not working, due to very big amount of users (members, member roles).
How can I do it? (Delete group)
And what is the relations on Group -> Project? My guess is Group has many Users, each User is a Member of a project, and each member has many Roles? Thanks in advance