There's also Backlog - a bug tracking tool that's free for up to 10 users. The tool works in a browser, you set up a project and add the bugs as issues in that project, and notify the users that you select. They can add comments etc. and so can you - so there's a thread of comments for the record and track resolution progress. Users you notify will need to be added to your account, and it's free to use for up to 10 users.