Q. How can I obtain the members of a role within a database?
A. One method is to use sp_helprolemember
EXEC mydatabase.dbo.sp_helprolemember --returns DBRoleMemberNameMemberSID My_Admin_RoleDOMAIN\muUser0x010500000000000515000000313CF1C8B4034LF676BJA5EE7C0D0000
For a comprehensive security audit on database roles – read Powershell sql server security audit