You can add a user by adding them as a member of one of your lists or folders.

To add someone to a list, open the list settings by tapping or clicking on the list name while the list is open, go to "Access" and click or tap "Add Member" at the bottom.

To add someone to a folder from your phone, swipe left on the folder, tap the "..." button, then "Access". 

To add them to a folder from your computer, click the three dots and then on "Access".

Type in their name and a complete email address. If they don't yet have an account, they will be sent an invitation to join Zenkit To Do. Once they have registered their account, they will have access to all lists and folders that you've added them to.