Sampling methods are generally designed to find a smaller group that is broadly representative of a larger group in order to be able to practically gather data from them. However, there are sampling methods such as opportunity sampling and snowball sampling that are simply designed to find people to study. A snowball sample works a little like a chain letter. For example, if you were distributing a questionnaire, you could pass it to your own contacts who could then pass it on to theirs and so on (the sample "snowballing" as a result). The resulting sample would not be representative of any identified target population but it is a way in which it is possible to access a large number of people.