Let user join an organic group during site registration I would like my users to be able to select a preexisting Organic Group to join when they are registering to use my site. Is this possible? How should I do this?

I already have the groups set up, and the permissions are set so that so that non members can 'Subscribe to group (no approval required)'.
You must enable the module:

Organic groups register (Allow subscribing to groups during the user registration)