The password must be at least 6 characters long. This password will be requested to join the group and register the participants. You will have to provide this password to all participants who wish to be part of your group if you do not want to register them yourself.
If you tick this box "Yes", you allow other people to join your group. After you have created the group, you set a password which you will need to send to your participants so they can register independently joining the group.
This password will be used to administer the group - Manage invitations, exports ...