When a hub is created, people are usually invited to join or directly put in by an administrator. If you would like to create a hub in which Members can choose to join, then this is the option for you. An "open" hub could be for an optional committee that anyone can join. Or it could be for people that need to coordinate something not directly related to the hubs they are in.
Once a hub is created, you need to go to Hub Settings and then the Joins tab. In the section Open Joins, there is a check box Allow any person in *organization name* to join this hub from the dashboard.
This will open the hub for anybody to join who is already registered within your organization. After you save it, a Join Hubs button will appear on the dashboard of every profile in your organization. This button will only appear if there are open hubs available to join.
On the Join Hubs page, a listing of available hubs will be presented with a join button on the right. The member can select the hub he or she would like to join. The hub will then be listed in the person's hub list.