To send the survey to specific leads you will have to set up a custom lead status to categorize the leads within and then send to the leads with that status.
To send the survey to specific customers you will have to set up custom customer groups to categorize the customers then send the survey to the customer group.
When you select into the Send Survey tab you will see checkbox options - Customers, Leads, and Staff. If you selected Only for logged in participants (staff, customers) you will not see leads as an option.
If you select Customers - you can send to all customers or select a customer group to send the survey to.
If you select Leads - you can send the survey to all leads, or select a specific lead status to send the survey to.
If you select staff, you will send the survey to all staff members.
If desired you can select multiple options (Customers, Leads, and Staff) to send the survey efficiently. Once you select who you are sending the survey to, you have to hit Send .