mGui is open source for a reason. If you'd like to contribute, we'd like to have you!
If you're interested in contributing but don't feel ready to make big changes to the code, helping us to improve and expand our documentation and examples is an excellent way to get involved. For examples, just submit a pull request. If you want to add a tutorial or work on the wiki, contact @theodox (or just open an issue to put your suggestion in front of other users) and we'll add you to the project so you can edit the wiki.
If you've spotted a bug or an oversight (say, a control is missing a new flag or has a typo), the best idea is to open an issue on the issues page first so that other users can verify the issue or help localise it to a particular combination of OS and Maya version. Submit a pull request referencing the issue.
So far we've generally evolved new features informally. Typically we'll open an issue to discuss the idea, then create a new branch for the feature and submit a pull from the branch as it matures. It's a good idea to start with an issue before diving in on development so you can hear from other users who may have tried something similar in the past.