When you create your hub or edit your hub you will see a whole block of information where your title is and the category choices you can make. At the top of this there are three tabs. One of them says 'Groups'. If you have 2 or more hubs that you have created as a series then you can create a group for them.
The entry is blank to begin with but you want to press 'new' and add a description that you want for the group. Once you have created this group you can select it for each of the hubs you want to have in the group.
Once you have added all of the hubs you want in the group and saved them you will notice at the bottom of each hub that there ae next and previous arrows to hubs that you set up in the group.