Henrik,
Of course you are more than welcome to contribute. I don't think that I have ever seen someone be turned away from Rainbow. It isn't just open source, it is an open community and anyone willing to put forth time is welcomed.
The best way for you to get started is to join the Sourceforge Rainbow-devel mailing list. This is the list that the regular contributors of the Rainbow project use to keep in touch and to notify the others with. It is a great resource.
http://lists.sourceforge.net/lists/listinfo/rainbowportal-devel
I would highly suggest that you just start by introducing yourself, share a bit of your background and mention what you are interested in working with. Don't be the guy with all the answers pointing out everything that needs improvement. Instead, start out with one area you are interested in and state a plan for improving it.
It is a truely awesome community. I'm sure you will really enjoy it, I know I have.
As for tools required. It is beneficial to have visual studio so that you can update the solution files. Of course, you could probably find someone to help you out on that. Other than that, you just need an editor(notepad, sharp develop, web matrix, notepad, vi, etc), a compiler(visual studio or sdk), and a sql server/MSDE database that you can use.
Oh, there is the code freeze, you will hear about that when you post to the list :)
Cheers
Chris Farrell
http://www.cftechconsulting.com