Here's your info, thanks to Rebecca Chernoff:
1247 (non-employee) users on Gardening
1122 registered users
1030 users have an account on SO or SU or SF
1003 users had SO/SU/SF before Gardening
I didn't ask for users who had accounts on SE 2.0 sites prior to Gardening, but as you can see from the above, that number is going to be pretty tiny.
They don't exist exactly as stated in your question.
There are badges -- Announcer, Booster and Publicist -- for sharing a link to a post using the share button underneath the post. (More information in the Q&A: How can I 'try' to get the publicist badge?)
On Area 51, you can get both reputation for each person that signs up for a site ...