When looking for images, I frequently use the "-everyone" designation.
However, I constantly come across images that seem like they should have the "everyone" tag.

Per the Wiki,

Images containing everyone (or a significant portion) of the cast in a show or game. This tag is independent of individual characters being tagged, so don't remove it.

which is fine and reasonable and easy to understand.

But what about images that have numerous characters that aren't "everyone or a significant portion of the cast of [a single] show or game?"

This image, for example: /posts/3183128

"group" is aliased to "everyone" but it would seem the two tags should have different meanings. I don't see any other tags that would indicate "a gathering of characters" that falls outside of the requirements listed for the "everyone" tag.

Well, mistags should be untagged, and there's bound to be a lot of those for big tags. In any case I don't agree with the alias of "group" to "everyone", imo it should be removed.

Well, I've been adding the "everyone" tag to suitable images, but I don't want to run afoul of someone who will angrily tell me that I'm wrong in doing so.

To find posts with many characters that aren't tied together by any specific connection, you can run the search chartags:>10 (or any number; or reverse that sign if you want to exclude those posts). Only specific connections like everyone or trait connection need to be tagged to be searchable.