Which method uses existing members of a group to recruit further participants?

The method that uses existing members of a group to recruit further participants is snowball sampling. This technique is particularly useful in research situations where the population of interest is hard to access or identify, such as specific subcultures or marginalized communities. When a researcher initially identifies a few members of the target group, those members can then help to recruit additional participants from among their acquaintances, effectively creating a "snowball" effect as the network grows.

In contrast, quota sampling involves selecting participants based on specific characteristics to meet predefined quotas, making it less reliant on existing group members for recruitment. Cluster sampling divides the population into clusters and randomly selects entire clusters, rather than individuals. Purposive sampling, on the other hand, involves selecting participants based on specific criteria set by the researcher, rather than relying on a chain of referrals following initial contacts. Snowball sampling is distinct in its emphasis on leveraging existing networks to facilitate the recruitment process.