We used to have a registration link on the IxDA website, but that's outdated and no longer active.
It was really just a means to get an account to post on the legacy IxDA mailing list, but that's been phased out and replaced with Discus, which is this discussions platform.
That's a long winded way of saying that you don't have to register anywhere to become a member of the IxDA. You can claim yourself to be a member just by participating in anything the IxDA does — whether it is being active on this discussion board, attending our local events and conferences, or participating in our many initiatives.
The philosophy of this is based around self-organisation and participation, which is largely how much of the IxDA operates across the entire organisation and community. There are no member fees, or certifications, or badges... just passionate people coming together around a common domain.
Does that make sense?