Manual Record Sharing
As a general rule, if org-wide defaults for an object are set to either Private or Public Read Only, then the sharing button will be displayed if added to the page layout. Some objects (such as account) may have the sharing button exposed depending on the sharing settings of related objects (e.g. sharing on the account can be used to share related opportunities and cases).
By clicking “Sharing”, a user can then manually share access to this record with other groups, roles, and users:
In order to share access to a record, the user must first have “Full Access” to the record (see Security Overview).
Viewing Record Access
The sharing button also provides an easy way to view who has access to record. Just click “Expand List”.
This gives a very granular view of who has access to the record: