What security should I double-check before opening my site to being multi-user? I've been using my D7 installation as a single-user blog, and operated assuming that would always be the case.  In the process, I may have been sloppy with permissions in various places, such as viewing, authoring, input formats, etc.---I'm no longer sure.

Now, however, I am going to open my site to subscribers (for a mailchimp mailing list), and possibly allow another content author.

Before I undertake these, what should I go over to make sure I'm safe, and ensure my site stays secure and is well permissioned?  Can someone help me construct a security checklist for all items I need to verify to move from single-user to multi-user?
Mittenchops, here is a suggestion


Create a role for the new users
Guarantee that this role doesn`t have permission to use PHP Filter 
Check all the "administer " permissions to see if the new role is not
having access to certain areas that it should not have
Create a new user for you with this new role and test the site.
During the process of creating the new user check if you are able to choose the role (you shouldn't be)
You should also try to access pages like http://yousite.com/admin which a common user should'n have permission


I think you would be safe with this.