Ask HN: How best to collaborate between designers and developers?

This is a question about productivity and task management software. I recognize that when it comes to topics like these everyone has their own opinions and there rarely is one "correct" answer. I'm looking for perspective more than anything else. This is my situation. In one of the organizations I contribute to, we collaborate very successfully using GitHub for code sharing, task management, code reviews, the whole nine yards. On top of this, we use Slack to manage private and group messages. This works very well for us; we communicate well, are rather productive, and friction between contributors is low. To top it all off, this group is a comfortable size (eight people) made up of dedicated, experienced individuals. The story for the other group that I contribute to is quite different. We've always used GitHub for code sharing, but we've never really tapped into its more powerful features for code review, issue and task management, etc. When it comes to interpersonal communication, we use Facebook almost exclusively, which has led to a general breakdown of focused discussion, which means that we easily fall off topic and little gets done. This group is larger and much more diverse. It's a student organization, so there's a strong emphasis placed on inclusion. Few more than a handful of members have any solid experience, though that by no means implies that they lack talent. Because of the emphasis on inclusion and diversity, this group has much more representation by artistically-oriented designers. In the past it has been argued (rather naively and without real deliberation) that we should abstain from services like GitHub for task management since these services are prejudiced towards programmers. The question then is this: for those people who feel that they've collaborated successfully on a team made up of both designers and developers, what, technological or otherwise, has best contributed to your overall success and cohesiveness?