Recently I was interested to recall when I was invited to the Gmail (my friend send one to me, when it was still a thing).
Seemingly, I've deleted some oldest emails back then.
So, I was invited somewhere in the beginning of 2007, as the oldest mail I still have is from 12th of February of that year.
Irony?
From Wikipedia:
"On 14 February 2007, invitations were no longer required to create an account and the service was opened for anyone to sign up for."