PS - if you use AlteredCart Checkout_One, add the <input type> lines in the file,
/skin/common_files/modules/Checkout_One/checkout_one_register_account.tpl
BEFORE:
Code:
<input type="password" autocomplete="off" name="passwd2" id="passwd2" size="32" maxlength="64" value="{if $login}filler{else}{$userinfo.passwd2}{/if}" onchange="javascript:checkoutDoPasswordsMatch();" />
INSERT:
Code:
<input type="hidden" name="membershipid" value="1" />
<input type="hidden" name="pending_membershipid" value="1" />
And then a new customer registering at checkout will automatically be assigned to your membershipid = 1 at checkout
IF you use other checkout templates, you will probably need to edit those.