Getting involved in Informl is as simple as letting us know what you think of it. You can post to the general discussion forum or you can email us directly. Try out the wiki on the sandbox.
If you're a developer, we are keen to get some people to help with making Informl better. Here's some areas where we would love to get some help:
Take a look at the "missing features" section of the features page for more inspiration on where you could pitch in.
Informal is also dependent on the Tartan project. It would help Informl a lot to get other mark-up types (Textile, Mediawiki, etc.) integrated into Tartan, and general performance improvements would also be really useful.