Create your customers/organizations in the system and add any discounts, notes, company info etc. You can subsequently add purchases to a customer so that they get their own history. Here you can provide an extra service for your regular guests.
1. Create Guest or Organization
Go to Modules > Customers
Press +
Press either Create customer or Create company
Enter a name and optionally add an e-mail and a telephone number (if it is a company, you can also add a CVR number)