One doesn't 'join' Spidell.  You purchase their various CPE offerings, and/or subscribe to their newletters, etc.

NATP is a membership; you have access to various CPE offerings (usually at a discount, sometimes free) as well as some other perks (discount on The Tax Book);  and you also get to ask research questions (the number depends on the level of your membership).  I have yet had them give a definitive answer that was at all useful.