Currently, there is no way of grouping multi-work series. You have to manually search for them.
Typically, it's for when a circle publishes a work instead of a single artist. It can also be for when a circle was part of a manga release instead of a doujin. You still want to tag all the artists that worked on a gallery, regardless of if a circle worked on it.
A circle is just a group of artists that decided to work together long term. Think of it like a band, the beatles for example. In a gallery the group tag includes the beatles, and the artists tags would include the members, John Lennon, Paul McCartney, John Harrison, and Ringo Starr, which would all get their own tag. However, they would only be included if they worked on that particular work, not just because they are a member of the group.
@akashako Cool. Thank you for the info. If that's the case, then I think it would be helpful if that description was included on the Groups or general info page.
I thought "groups" might have been used as a way of grouping the different galleries that were part of a multi-work series.
So is the idea that a user would use the groups tag instead of a specific artist tag because they want to find works produced by a specific circle. Does the circle typically represent a creative team of artists collaborating on a project, or is it more like a group of artists coming together as an independent publisher?
Groups are for circles, groups of artists that officially work together. For example https://nhentai.net/artist/rebis/ and https://nhentai.net/artist/chinbotsu/ formed a doujin circle called https://nhentai.net/group/arsenothelus/