Is there a diagramming tool that allows you to edit diagrams in Confluence with several people at the same time?
Yes, the draw.io app recently enabled simultaneous diagram editing in Confluence Cloud. You can read more about the live collaboration feature and find out which workflows it’s particularly useful for in this blog article: How to improve your Sprints, Real-Time Live collaborative diagramming
Visit the blog-page for more great stuff on draw.io in general!