There are several places in the system where you are allowed to search for a membership or group record while you're inside of another record. For example, if you're adding members to a Group, or associating an existing member to another member.
This is the quick search window for selecting a member.
The fields available for searching are limited to some key fields based on the type of record for which you are searching. In this example, key name and address fields are provided to help find a member.