Public groups in the sharing model
Key Takeaway
When users who need the same access don't line up with a single role, put them in one public group and make the group the target of a single sharing rule.
A group is a set of users that you can use as a single target when you grant access in Salesforce. Administrators and delegated administrators build public groups to share records, folders, list views and other items with users who don't line up with a single role. Because the whole group acts as one target, one sharing rule can do the work of many.
Public groups and personal groups
There are two types of groups. Only administrators and delegated administrators can create and edit public groups. Any user can create a personal group for their own manual shares.
Personal groups are available only in Salesforce Classic, not in Lightning Experience. So when a scenario needs a reusable sharing rule target, folder access or support for Lightning Experience, it calls for a public group.