If you elect to on the form, you will be subscribed to the aigor mailing list, which is really the heart and soul of aigor. This means that you will get maybe 20 or 30 Guzzi related messages per day in your inbox, though it can be up to 50 per day at times. Otherwise you will only receive an occasional administrative message. You will receive instructions for removing yourself from the list if you want to. If you want to know more you should read the welcome message. This in turn will ask you to read the listFAQ, which is essential if you want to get the most from the list.
The other thing is your details get entered on the members page. This is the owner's register part - a reference for people (members or not) who want to contact other Guzzi owners in their area, or perhaps other owners of certain models. You can just be on the members page without being subscribed to the mailing list, but it won't make you feel a part of things like participating in the mailing list will.
This 'half' membership is under review. The problem is that people register, get freaked by the mail volume and drop off the list and we never hear from them. They move and forget about aigor and the members page is left bloated by vestigial entries. One option may be to require non listers to re register once a year or lose their entry.