pre-shared key option disappears when I select "peer ID from dialg group"

Hello, I'm trying to set up a VPN connection. At the moment I want to limit access because anyone can connect without a username or password.

My users are in a group and I want to select the same group in order to authenticate. When I choose the option: "peer ID from dialg group"  the pre-share key option disappears and I can't save because it gives an error: - 1 invalid lenght of value.

Can anyone help me? I just want to authenticate with a list of users.

Thanks.