No, although reach does depend on who sees what. For a closed, secret group, there would be no variations. But for an open page, sure, as you and the other admin have friends who are not members of the group and, if you (or anyone else in the group) interacts with a post, it ups the reach.
Start reading the Buffer blog, where they investigate this stuff for a living:
In the interests of full disclosure, I am not an employee of Buffer.