I want to run a project with my girlfriend and have us both accessing the task lists and commenting...adding info...tasking each other. Is this possible?
Unfortunately, that's not possible right now, but we may consider adding such option in the future. You're welcome to vote for this request in our Votebox: http://todoist.com/Vote/showProposal/204/
For now, you could try to use our other product - https://wedoist.com a team-based project management tool allowing you to invite people to projects, directly assign tasks, post comments, status updates etc.
The only option to add a task to someone else's Todoist account is through the "Email task to this project" feature. It's only available with Todoist Premium and enables sending tasks via email to Todoist projects if you share the unique email address of your project and add multiple accepted email addresses to your Todoist account.