I recently discovered https://hacknplan.com/ and are using it right now. It has free and paid solutions.
It was specifically made for game development and also has Game Design section, that I am not using.
It is limited and still in development but much easier to use than Jira for example.
I also own Jira and especially confluence is a great tool for documentation, requirements management and so on,
But just for simple and interdisciplinary task management I prefer HacknPlan, Trello or Asana.
Also in general you don't have to invite each member to such services. I would not invite peopole that just to a few tasks. You can track their work on your own and send them tasks by mail or whatever.
