Be sure to check the official  PlantUML docs  to check what you can do, and choose a unique session name to avoid unwanted collaborators

Just share the URL with the people you want to collaborate with.

Be aware that anyone can go to any board, hence, avoid placing private information.
Or get a CollabUML installation specific for your company on so that you don't have to worry about private data,, plans starting from $499/month.