Have Square on my iPad. Want to add it to a phone for a large offsite weekend sale.

[Seanzenp]

Seems to work but now asking me to log in using a passcode. How do I get the passcode or get past this?

[BofBArtStudio]

Passcodes are set per user in your dashboard.  You can click the user and scroll to the bottom to see the passcode.

You can also create permission sets per user to allow/disallow certain areas of the POS on phone or ipad or dashboard.