In case the client is switching over from WordPress or some other CMS to Squarespace, the client is then added as an administrator. This is after they approve the site by clicking over Settings>Permission>Invite Contributor.
First, they create a Squarespace account. This lets them log in to the backend of the site. Here, they can make an upgrade to a paid site from a trial site. The billing information can also be set up over here.
For more detail click here: https://www.cyblance.com/squarespace/squarespace-design/