SharePoint - can't submit a Designer-created form (Access Denied)

So I have this site to which a given set of users have read access. I created two lists--one for a list of available classes, another for sign-ups--and granted the users Contribute access. I created two forms--one to sign up, another to manage sessions--and gave the users Contribute access to both.

When I sign in as one of the standard (non-privileged) users, I click on the sign up button and get Access Denied. If I go to either of the lists as this user, I can create a new item. If I grant the user Contribute access to the entire site (which I don't want to do for all users), I can submit the form with no error.

I'm kind of at a loss as to what could be at issue; I'm only using those two lists and those two forms in the process, and the workflow isn't creating Tasks or anything like th开发者_开发百科at. Please let me know what other clarifying info I can provide, and thanks in advance for any help you can throw my way :)

-AC

It seems the default "Visitors" group has read access to the Workflows folder; this is what's required for the workflow to complete.

Now if I could only figure out how to grant other groups access to the Workflows folder...