> I wonder if there's much history of groups starting out with common languages, then groups separating and the languages evolving separately, either intentionally to demonstrate group membership, or just due to lack of communication between groups, and continuing to differentiate until they're effectively different languages.

Thats basically what sociolinguists investigate. Here's an example of a study that kind of does that: https://www.researchgate.net/publication/233713880_A_Quantit...