UpStage is an open source software development; we welcome any interested developers to join the team. You can:

Developers and artists meeting at the 10th Birthday celebrations, 2014.
Developers and artists meeting at the 10th Birthday celebrations, 2014.

Note: previously we used Sourceforge, and there is still some information and history of the project accessible here. But this is not up to date and anything relevant has been migrated to Github; please see the wiki on Github.

Developers’ blog