From old, the’ve requested that bugreports sent via mailing list, but if you are familiar of the process and get more involved, there is the actual dev system and the PureOS issue tracker, where you can make your reports (first is directly about the new Crimson, latter is a bit more broader). However, the registration to that may need you to contact the Purism team as automatic registration was closed due to spam. For more general testing, use the testing thread where you can report changes in behavior and features and edit the table yourself (good place to ask questions if your encountering odd behavior that you’re not sure is a bug).
Btw @JCS: The documentation text about this could be expanded a bit, to include more “how-to” and “tips on good/informative reports”. [This could have been a ticket 
]