Assign to user group by two different date values?

You are right, I wasn't clear: how do I use / where do I set
date( 'FORMAT', '[cb_date]' ) as condition rule?
I am not quite sure where to set it. Otherwise I got your answer right.

What I am doing is:

at registration I offer a field called birthdate. The entered date, let say 1976-09-18 should be stripped and be compared to the date value of "virgin", which is between yyyy-08-24 and yyyy-09-23.
The date fields for "virgin" are not displayed during registration.
So what I need to know is, how to make the match between 1976-09-18 and 2012-08-24/2012-09-23, 2013-08-24/2013-09-23, 2014-08-24/2014-09-23 - maybe it is described a little more clear now. If there is a match, the user should be assigned not to registered but to the group virgin.

You are right, I wasn't clear: how do I use / where do I set
date( 'FORMAT', '[cb_date]' ) as condition rule?

You don't, it doesn't exist yet. That's a feature ticket for next release.

at registration I offer a field called birthdate. The entered date, let say 1976-09-18 should be stripped and be compared to the date value of "virgin", which is between yyyy-08-24 and yyyy-09-23.
The date fields for "virgin" are not displayed during registration.
So what I need to know is, how to make the match between 1976-09-18 and 2012-08-24/2012-09-23, 2013-08-24/2013-09-23, 2014-08-24/2014-09-23 - maybe it is described a little more clear now. If there is a match, the user should be assigned not to registered but to the group virgin.

You're probably going to need to use a CB Query Field and do a database query for the field then do your comparison stuff there using MYSQL functions as the detailed comparison you need the conditionals can not do. You'd do the comparison in a query field with MYSQL then output a 1 for true and 0 for false then just check for 1 or 0 in your conditional.