It is my understanding that only organizational members can add custom fields. If this is true and organizational members have to have the same domain is there a way to make an email that is not the organizations domain a member for full rights in regards to custom fields.
You’re correct, only full members are able to apply/edit/remove custom fields of a project.
Guests who have access to the Tasks within a Project, on the other hand, have the ability to apply the custom fields that have already been selected by a full member to appear on the Task within that project.
If you wished to give a Guest the rights of a full member, they must be given an email address registered at the domain tied to the Organization, which then needs to be added to their Asana account by following the instructions available here.
Hope this clarifies things a bit!