Adding one
Customers → add a contact with at least a name. An email address matters more than it looks: it is where booking confirmations, cancellations and receipts go, and without one we decline to send rather than guessing an address.
Bringing a list with you
If you are moving from a spreadsheet or another tool, there are two importers and they are not interchangeable. Customers → Import is the one for families with children: one row per child, and rows sharing an email address or a family name are folded into a single household. Only an Owner or a Manager can open it.
For a plain list of adults — names and email addresses, nobody's children — use Customers → Contacts instead. Bring names and emails first; the rest can follow.