Currently, each login is private for each user. While it's possible to share your visual with others so they can view it, they won't be able to edit it. For editing, visuals are always linked only to the account on which they are created.
Each account registered is by one person with their email address, and we do not provide a multi-user account option at this time. We're currently working on Piktochart for Teams, so let us know if you're interested in early access!