The "Musician" User Group Only Allowed Within Custom Fields

When you are adding custom fields you will notice you can only select the user group "Musician" from the "User Group" selection drop down. This is because the "Musician" user group is allowed to have custom fields, while other user groups by default do not have this option enabled. This feature can be enabled by editing the settings for each user group and enabling the option:

Can have special custom fields?