I noticed in the checkout process you can checkout without being a member.
How can I prevent this? I need anyone purchasing to have the option to sign in if they have an account or sign up in the checkout process.
You can use the output of the islogged in module and then simply hide the add to cart buttons on products and have a note saying "You must be logged in to purchase this item" or something similar.
Here are some useful tips: http://kb.worldsecuresystems.com/654/bc_654.html, with one that includes checking for the module_isloggedin.