By default, groups here are set to private. This means that the person who created it (ie, the admin of the group) needs to grant permission to other members to allow them to see the content.

As the group admin, you can choose to make your group Public. If you do this, anyone can join the group simply by requesting membership. No approval needed.

Only members who have joined a group (and site admins) can see the content.

No matter what setting you choose, only people logged in to the site (ie, have a member account) will be able to see any content, so choosing Public for your group may be the easiest path if you want a lot of discussion.