How To Add A New User (Squarespace)

1. Open the Permissions panel and click Invite Contributor.

2. In the Invite Contributor window, enter the contributor's name and email address.

3. Switch the toggles to give the contributor permissions based on how they'll contribute. You can give them Administrator permission.

4. Click Invite to send your invitation.

5. Your contributor will receive an email invitation asking them to log into their existing Squarespace account or create a new one.

6. Invited contributors are listed under Invites Sent.

Reference: 

https://support.squarespace.com/hc/en-us/articles/206537287-Inviting-a-contributor

How to provide full CMS Access (Squarespace)

Administrator - Administrators can do almost everything the Site Owner can do, except change the site ownership. This is a good permission level for collaborators who need full access to your site.