Yes, you can register a member using the “Member” module in a template. Then an email link or a form post could create a new member.
It looks like maybe {exp:member:register_member} will work. But I don’t know. This post may have more info: http://ellislab.com/blog/entry/an_extension_autopsy_from_dr_jones/
😊
Here it is, I was making it too complicated: http://expressionengine.com/wiki/Standalone_Registration_Form/
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