If you want to get members of database role you can use the fallowing query:
select Users.name as UserName, Roles.name as RoleName
from sys.database_role_members drm inner join sys.database_principals Users on drm.member_principal_id = Users.principal_id
inner join sys.database_principals Roles on drm.role_principal_id = Roles.principal_id
where Roles.type = 'R' and Users.type <> 'R'
If you want to get the members of an active directory group members in order to see who can login to the server you can use the procedure xp_loginfo.
Adi