To ensure that the membership rollups in NPSP (Nonprofit Success Pack) work properly for managing a membership program, the consultant needs to configure the following items:
Select the membership record type for membership rollups:
This involves defining which Opportunity record types are used for membership donations. This configuration allows the NPSP to properly identify and roll up membership donations.
Reference:
Create an Opportunity record type for memberships:
Opportunity record types allow different types of opportunities (such as donations, memberships, etc.) to be tracked and managed separately. Creating a specific Opportunity record type for memberships ensures that these are properly categorized and can be used in rollup calculations.

(https://powerofus.force.com/s/article/NPSP-Configure-Addresses#topic-2559)

To provide each affiliate and the national nonprofit the ability to see only its own donor data in a single Salesforce org, the consultant should use Criteria-based Sharing. Here's why:
Criteria-based Sharing:
This feature allows for defining sharing rules based on specific criteria, such as record ownership or certain field values.
By setting up criteria-based sharing rules, each affiliate and the national nonprofit can be restricted to viewing only the records that meet the specified criteria, such as records tagged with their specific region or affiliate identifier.
Reference:
Salesforce Sharing and Visibility Guide
Salesforce Criteria-based Sharing Rules Documentation