There's a somewhat small, but still quite active community of editors who deal with doujin music here on MusicBrainz. I'm one of them - I mostly work with stuff from the Touhou community, which has some particularly interesting things to deal with…
In general, Circles seem to fit best as Group artists in MusicBrainz. A few examples that we already have in the DB: On the Touhou side: ZYTOKINE, a solo circle by 隣人; Alstroemeria Records (famous for their take on Bad Apple!!), which is Masayoshi Minoshima's circle, but also has several other members. For original works, Primary, which is yuiko's circle, but has some recurring composers, and binaria which behaves more like a traditional band, with several constant members and fewer guests. On the side of something more like a project manager, Diverse System puts out a large number of albums featuring a variety of individual artists, following a theme.
In all of the cases I've mentioned, the Group artist is the main way that we handle the circle. But, as the Circles are also self-publishing, they sort of act like a label at the same time. As for the label, there's basically two competing way to handle it:
- Use the special-purpose label [no label], which is the standard way to handle self-published releases.
- Create a Label for the circle, in addition to the Artist. E.g. Someone's made one for EastNewSound, who are also the artist EastNewSound. This has generally been very inconsistently applied. Every circle is an artist, but only some have labels.
As for the artist credits on the release, that's also something that's hit a bit of contention now and then!
We almost universally use the circle artist as the Release (and Release Group) artist. But there's no agreed-upon standard for what to use for Track and Recording artists. I generally use individual artists if they're clearly listed as single artists in the track list. For Touhou arranges, where the arranger & vocalist have similar importance, I usually fall back to the circle name. In some cases, I see track lists that have "Some Song feat. An Artist" but no main track artists - in that case, I usually do "Some Song" by "Circle Name feat. An Artist" in MusicBrainz. Other editors may vary in the style they use.