The third tab in Hub Settings is the Joins tab.
To get to the hub settings, click on My Hubs on the upper left> Select the hub from the drop down > click hub settings.
Click Admin Console > Hubs > Then select the gear at the end of the row with the hub you would like to edit.
Once you are in the Settings, visit the Joins tab by clicking on Joins.
From there, you can:
Put in a Description about the hub to display on the Join Hubs page or Public Join Page.
Check this box if you would like the hub to be open for anyone on your MemberHub site to join via the Join Hubs page.
Lastly, you can use adjust the settings of the Public Join Page for your specific hub. You can choose if there is a public Join Page turned on, If you require Admin Approval for Joins through the page, edit your Hub Key, and upload a custom photo for the Join Page.
When your done editing these changes, be sure to click the blue Save Changes button.