A criteria-based sharing rule is a tool that allows administrators to share records with certain users based on field values rather than ownership. For example, a criteria-based sharing rule can share all opportunities with a cross-sell opportunity type with a specific group of users. A public group is a collection of individual users, roles, roles and subordinates, or other groups that can be used to simplify sharing settings and other processes.
In this case, the administrator can add the subject matter experts to a public group and give them access to records with a criteria-based sharing rule that matches the cross-sell opportunity type. References:
https://help.salesforce.com/s/articleView?id=sf.sharing_criteria_rules.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.public_groups.htm&type=5

A roll-up summary field is a type of field that calculates values from related records, such as the count of child records or the sum of a field on child records. In this case, the administrator can add a roll-up summary field to the open house object to track the number of visitors by counting the child records on the visitors object.
References:
https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.htm&type=5

Section: Volume J
Explanation/Reference: