I use Github and Riot for work, so you will need an account on both platforms to collaborate on my published projects. Sign up for GitHub here, and Riot here. For Riot, create your account on the matrix.org server.
The private chat is here. You won’t be able to access it unless you have an active subscription and an invitation to join the room. I review the chat to discuss ongoing development and support details. If for whatever reason you have a subscription but do not have your invitation, please email me at moc.drargvxufegegas@egas.
To open issues, you will need to use the GitHub issue tracker for the project you care about. Depending on the conditions and urgency of your request, it will be assigned a support level. See here for definitions on what levels exist and when they apply.