Firstly, you could vote for this feature request Create a new taxon rank for iNaturalist: the species group - Feature Requests - iNaturalist Community Forum
We had a similar issue with the large Hoverfly genus Platycheirus (circa 200 species). In this there are taxonomically recognised subgenera, which in turn are divided into well known species groups - but there are also smaller groups that are widely used and fall within these larger groups. We used the rank ‘Section’ to represent Species Group and Complex for the subgroups. Which is a bit of a fudge, but it at least makes (almost) all the well used divisions available. You can see the discussion here Flag for Taxon: Section Platycheirus albimanus · iNaturalist
One thing to add though, the guidelines do suggest not adding additional taxonomic nodes unless they are genuinely going to be useful, not just because you can.