My (wholly unoffical) take:
- Start with bugs. Find, report, fix, create a differential and submit.
- Appoint a Geometry Project member or three as patch reviewers.
- Finding bugs are easy - try and use Geometry Nodes in some useful way and something will surely break.
- Familiarize yourself with the Geometry Nodes discussion, where there has been much praise and some gnashing of teeth. The latter hints at bugs.
You won’t have to wait for an invitation or ask permission to join because nobody will ever tell you not to try and fix a bug. Bugs usually have a compact scope. You can wrap your head around a bug easier than a gemungous thing like a bevel modifier. Bugs give rise to pretty directed questions that can be asked here or on blender.chat. Maybe the reviewers won’t like your patches, and if they don’t, they’ll provide reasons why not, which lead to greater insights on your part. Keep at it and soon you’ll find yourself involved. Have fun!