We do not recommend using the group key as the administrator key, as it would give all of the users of that key administrative access to the encrypted folder. They would all be able to add or remove other keys and potentially block other users (or even the entire organization) from accessing the files.
I would instead recommend generating a new keypair to use as the admin keypair. Then encrypt the folder to that keypair, and the group key. Store the admin keypair on a flash drive in a safe location. Since that admin keypair is not bound to a specific user, it could be retrieved by the IT staff that have physical access to it if it was needed for anything, but it should not be needed, as the group key would grant access for anyone in the group.
Once a user is removed from the group, their access would be revoked, and coupling that with NTFS permissions is the best way to keep the share secure.
If for some reason, all of the end users were completely trusted to have administrative access to the folder, the setup you described above should work, but I would not recommend it.