I added a “Reason for joining” custom field, and put a checkmark on “Required at signup?”. That works well for those that sign up for an account without an invitation. With an invitation, the account is created and the custom field is left blank. So far not a problem.
But when the user then wants to change his/her profile, that empty custom field blocks saving. It is not easily seen why saving was denied, and the error message is also not pointing to the custom field. Also, there is no need anymore to require that field.
Is there a better solution to require new members to declare a reason for signing up?