Once you have created a product and the payment form added go to Products and select your product.
Go to the Assignments tab.
Select Assign members.You may filter to specific teams, and roles to assign specific groups of members.
Prior to creating your assignments you have an option to send payment requests by selecting the tick box above the filters.
Once members are selected, the number of assignments will display at the bottom on the list.
Once you are happy with the members you have assigned select Create X assignment(s).
You can now also assign products to certain members when creating them.
When creating a product you will be given the below options:
This allows you to specifically target certain members, assign the product to everyone, or make the product unavailable to purchase.
These settings can be changed later in the product settings.