[Zope] bypass authentication

I made a signup form for my website and I want to check if the username already exists in acl_users folder without having to log in as administrator. How do I do that, what and where do I have to modify my permission for so I can use acl_users.getUser(REQUEST.name) without having to login first?? (Zope tells me that I don't have permission to use the getUser method)