The third tab in Hub Settings is the Joins tab. Here you are able to make the hub available to be an Open Join Hub. Read more about Open Join Hubs
You are able to put in a Description about the hub. You can check the box if you would like the hub to be open for anyone on your MemberHub site to join!
Lastly you can use the Public Join Page for your specific hub. By posting or sending out the link, people can join your hub directly. Make sure to click on Save Changes!