Rollbar uses a default grouping algorithm. We try to group occurrences into items that have the same cause to reduce noise for similar occurrences. We choose to group with the exception class name over the exception message because the exception message often varies for each call. We hash the filenames and method names from all of the stack frames plus the exception class name to create a fingerprint. All occurrences with the same fingerprint are grouped together. You can read more about our grouping algorithm here.
You can adjust Rollbar's default grouping behavior by setting up custom fingerprinting rules. Take a look at the docs for custom fingerprinting here. You can create rules, which are a JSON object made up of a condition, fingerprint, and optional title. You can use the exception message in the fingerprint so that occurrences will be grouped by exception messages, rather than by exception classes.