some quick ideas - telegram group (you can pin messages and some other features that whatsapp doesn't have) - mumble server with push-to-talk - trello board

some quick ideas - telegram group (you can pin messages and some other features that whatsapp doesn't have) - mumble server with push-to-talk - trello board - etherpad / google docs - possibly syncthing (if you're dealing with files and don't change them too much, be careful with conflicts)