To start, hit the "plus" sign here:
This will reveal the Saved Customer Search module:
Here you can click "New Search" to create a new customer list:
You can enter a Search Name, select the criteria for the search from the list below, then save the list. Once saved, you'll get a count of how many customers are in the list.
You can then click that search anytime to pull up that list from the Customer page, you can export to a CSV for use elsewhere, and you can also use this as distribution list in the Marketr module.