at the top of the page, you can just go to Editing → Add Series, set the type to Release Group Series, and add the individual release groups.
you can also add it from the release Relationships Editor (the “Edit Relationships” tab on the release page) or editing a release group directly. if you do use the Relationships Editor, at the bottom of the page there’s a place for release relationships and release group relationships.
a subseries can be used if you’ve got say, several related series of different types or a couple distinct series within a series. for example, my K-ON! release group series has subseries for the two character image song series, an audio drama series, and a series of Yaroyo (albums with music score included). everything from the two anime seasons and one movie is directly in the main series. again, we as a community are still trying to figure out how best to use series, so this is by no means a standard way of doing it 
actually, another way to link things in a not-so-structured way is with tags (aka folksonomy tags). you might have already found the hololive tag, as a good example, but you can add tags for literally anything.