How to make a group show only on first visit?

[Jici]

You can create a field Yes/No in User DB. And update current user. When user is not logged in, this update will be store in cookie. However, Cookie have a limited lifetime (3 days I believe).

You can also consider using your own cookie script instead.

How to attach anonymous data to existing user after they log in?

I ran into exactly this recently and solved it with a simple cookie (read on for deets). As you note, for new users it works fine to save data to the “Current User” which then gets attached automagically when that anonymous user converts to a registered user – at least within the lifetime of the cookie/anonymous user in the database [something like 3 days, I believe?]. However, this does not magically work for an existing registered user who visits your site, does some stuff, and then later in…