Administrators
Invite whoever is helping you prepare the wedding.
Invite someone
In Settings, under Administrators, type the person's email and press Invite. They will get an email to accept.
They have to accept with an account under that same email: if they do not have one, they create it there and then.
An administrator can edit the website, the guests and every other section. A wedding can have up to 6 people between the owner, administrators and pending invitations.
The owner
Whoever created the wedding is its owner. Only the owner can invite and remove administrators and switch on card payments for the gift list.