You can see the logins available from the system view sys.server_principals. You should not/cannot remove logins by removing entries from that view. You should use the SSMS user interface or a T-SQL query (DROP LOGIN) to remove logins.
Before you remove the logins, remove the database users associated with those logins. Otherwise, the logins will become orphaned. Then, when you remove the users, any permissions you granted to those users will be gone along with that. So after you create the windows group login and corresponding users, you will need to grant permissions to those users. A better alternative might be to create database roles, grant permissions to those roles and add the groups to those roles.