Penicillium is one of the biggest genus of fungi with more than 350 recognised species and more being described every year. In 2020, Houbraken et al. sorted the infrageneric classification of this genus by creating subgenera, sections and series. However, series are not supported by iNaturalist. As an example, I took the time sort the section Exilicaulis to an older way of referring to them as complexes or clades (https://www.inaturalist.org/taxa/1148246-Exilicaulis). The biggest series of this section is Series Erubescentia here referred as Complex Penicillium erubescens with 18 species. A way of naming these complexes would be to use the name of the type species of the series as I've done here. However, many series are monospecific such as Series Digitata (with only Penicillium digitatum) or Series Osmophila (with only Penicillium osmophilum) and wouldn't respect the definition of a complex.
Following the guidelines of the Curator Guide:
1 - Species complex is monophyletic (i.e. sibling groups of species)
This would be respected in some cases. However, some species would quite distant and many series would be monospecfic.
2 - Complex is recognized in the literature
Complexes views have changed in literature so it would be respected only with older literature. Now complexes are viewed more precisely.
3 - A named subgenus, section, or series does not already exist for the group
It doesn't work as all would have names. But no alternative exists for series.
4 - If a "principal species name" is not established in the literature, use the earliest published species name for the name of the complex.
As stated above we could use the type of the series.
5 - Don't use compound names, such as Pantherophis alleghaniensis-spiloides, as there may be numerous species in the group.
This would be respected.

This problem is actually the same in Aspergillus. I'm also certain Talaromyces will follow sooner or later. Other fungal genera have the same problem such as Amanita:
https://www.inaturalist.org/flags/525830
https://www.inaturalist.org/flags/517547
https://forum.inaturalist.org/t/implement-the-botanical-rank-series-between-subsection-and-complex/17735/21