If a Group is not shared during it's creation, you can share the group directly from it's interface by selecting the appropriate sharing options and specifying the recipients.
Go to the Groups section and view all available Groups.
On the group you wish to share, click on the View members button to see all current group members and their role (Admin, Editor, Viewer).
To invite more members to your group, simply click on the Invite (+) button.
To share directly with individual users enter the email address(es) of the user(s) you want to share the group with and define their access level (view only, or edit access). Click 'Save'.