For every research project, you have to find the right users who will most likely tell you what you want to learn.
Even if you have a ready-made list of customers that use your product, you still have to find the right users within that customer group.
I usually do a screener as part of the overall test plan. The screener for most rapid user research can be a list of attributes that the users must have or what mix of user types are needed for the study.
Here is an example of a quick screener. Note the questions that I ask in RED. All logistics have to be agreed and planned in advance.
A more detailed, formal screener for a study on a re-design of Comcast.com.