First, you must join a [User Team] and ensure the user you want to share the group with is also in the team.
Then, on the [Groups] home page, you can find the groups you own in [All Groups], [Auto-updates], and [Snapshot]. Next, click [More], then click [Share].
In the window that appears, select the user you want to share the group with. You can choose more than one user. Finally, click the [Share Group] button to complete the setup.