Mixtable's Excel-like spreadsheet interface is the perfect way to import customers to your Shopify store. You can add customers individually or in bulk, and you can manually enter the customer information or paste it from Excel.
To get things going, add a new worksheet that is set up to sync with your Shopify store's customers.
The worksheet comes preloaded with columns for the most commonly needed customer fields. You are free to delete columns you don't need (right-click on the column headers to see the delete option), or to map blank columns to sync the data you need. To make a column show a particular field (e.g., the customer's email) you need to map a column to the field. To do so right-click on the column header and select Shopify Sync Settings.
In the window that opens, you have a choice whether to map a Core Field, a Metafield, or a Mixtable Analytics field. Once you've selected the field you want to map to the column click on Save and you'll see any existing Shopify data start to populate in the column.
Now that you've set up the columns you need, it's time to add your data. You can either manually enter the information, or paste it from another spreadsheet application (e.g., Excel). All the cells that you modify will get a light, purple background. This denotes that these cells' contents has changed, but hasn't been "committed" yet. When you commit the cells, Mixtable will sync their contents to Shopify.
Once you are done entering all your customers' data, and are good for the data to be synced to Shopify, simply click the Done Editing button in the upper-right-hand corner. This will "commit" the changes, and trigger their sync to Shopify.