You can share files from your Keep&Share account with anyone, whether they already have a Keep&Share account or not. In order to share with someone that does not have a Keep&Share account, you will need to paste that person's email address to the Share Control of any application in your account.
If you paster someone's email into the Share Control, that person is then issued a "Guest Account." This is an account with limited features that allows someone to log in and see things that you have privately shared with them. In a Guest Account users won't be able to change the email associated with the account, use any of the Keep&Share applications in their account, or share anything.
If you would like to have access to these features you'll need to upgrade your Guest Account to a free Basic account. This is still a free account but it will allow you to have more features and control over your account. The free Basic account won't send you any more email than you're already getting from Keep&Share and it won't be any harder to use. Upgrading your account will just give them access to "My Account", "Account Settings", and "Communication Settings."
First make sure that you're logged into your Keep&Share account. Then to learn how to switch your Guest account to a free Basic account follow the steps in the image below: