Hi Tomasz,
Glad you have joined the Pike developers!
To contribute something to Pike you need to first read and comply with http://pike.lysator.liu.se/development/git/policies/. The part that probably requires most thought from your side is the "Pike copyrights" point. You have to have the legal right to relinquish your or Operas copyright claims over the copy of the code you want to contribute and give Linköping University/IDA the full right to do whatever with it. This includes changing the license in the future without consulting you. That is not just a theoretical possibility, the code used to be GPL2 only at one point, but it was changed to GPL2/LGPL/MPL.
You retain full rights to your copy of the code and can do whatever you want with it without consulting IDA.
When I was an employee with Opera Linköping we had blanket OK to do this, but you might want to double check with your manager or talk to Tobias tobij@opera.com at the Linköping office who might be better updated.
No signing is needed, just make sure that you don't get yelled at internally and then reply to this say "I've read and will comply with the contribution policies".
When the formal stuff is out of the way, make a pull request against https://github.com/pikelang/Pike and send another mail to this list pointing to the PR URL. We so seldom use Github PRs that not many people are monitoring them. Normal development is on feature branches on the main repo and then you merge them yourself when the general consensus is that it's a good change. Lets discuss setting up that access for you (if you want) after handling this as a PR.
Regards,