I tried to figure this out for you but could not. If you joined a group when you first joined Nexus, the group moderator can look up the date you joined their group - that's easy. That's if the group moderator is active. You could message them.
I took Sentient Biped's advice and filled out a message at this link: http://www.atheistnexus.org/about/contact-information
We'll see if I get a response. If I do, I will share it here.