The Android app for TrackQueuer https://trackqueuer.com/
TrackQueuer is a free-to-use application which lets other people add songs to your Spotify queue, because listening together is even more fun. Only one user has to download the TrackQueuer-app from Google Play and you're good to go. Share the link with your friends and let them add their favourite tracks to the queue, and the app will synchronize it with the Spotify app.
Yes, you definitely can! If you have experience with Android and/or the Spotify Android API, there often are some issues you can help with. Click on the "Issues" tab above to see open issues which still needs to be solved. These also contain new features. Just fork the project, create a fix, commit it to your fork, and then create a pull request so I can review your changes. I'll try to look into your pull requests ASAP.