When trying to find contacts who have memberships matching assorted criteria, you can use Search Builder to find contacts who are listed in the Member field, or the Member 2, 3, 4, 5, 6 or Billing Contact fields of their corresponding membership record. See screenshot below:
When you are setting the search criteria in Search Builder, there is a drop-down menu that allows you to tell Sumac which Member Contact field you want the criterion to consider. By default, it is set to "Member," but if you open that drop-down menu, you can set it to look at Member 2, 3, 4, etc., or the Billing Contact instead. For example, when trying to narrow in on any contacts who have Family Memberships, set a criterion that looks at the Member Contact, and create a duplicate criterion that looks at Member 2, another for Member 3, etc., then combine them with an OR connector.
This approach allows you to find all contacts who may be linked to a membership for a more complete final list.