New Post: Create a public guest account without registration?

I’m currently setting up BugNET 1.5 as a public and internal bug tracker.
It is required that any visitor of the site can create issues (and add comments but nothing else) without registration (guest role) as I know that most of our users are just too lazy to create an account.
While it is easily possible to set a project’s access type to public and also to create such a guest role with the appropriate permissions I just don’t know how to give this role to every visitor of the site automatically. Is that possible at all?

Thanks in advance!