How do I close a form and reopen it on another computer to complete the form before I email it?

Simply logout, and log back in the other computer. The form builder saves every single change in your form automatically, once you finish building the form, then you can email it: https://www.jotform.com/help/303-How-to-Send-a-Form-via-Email 

If what you meant was filling your form, and saving the filled data before submitting it, then you can apply the following workaround: https://www.jotform.com/help/97-How-to-Save-Forms-and-Continue-Later 

Let us know if you need more help, we will be glad to assist you.