Public Groups are the most flexible method to grant access to an unrelated group of users. Public Groups can include individual users, roles, and roles with subordinates, allowing for granular control over which users gain access to records. Sharing sets and role hierarchies are less suitable because they rely on predefined relationships, such as account ownership or role-based hierarchies, which may not apply to unrelated users. By leveraging Public Groups, administrators can define and manage record-level sharing without requiring additional custom logic. (help.salesforce.com)