The profile of the user must have read and modify permission. Further, OWD must be set to private and sharing rules based on region must be in place. So user will be able to see his own and all other records related to India.
With sharing can be used in an apex classes which inherits all the sharing settings and rules for the class. This means the class may only access the records which comes under sharing criteria.
Records can also be shared using apex programming. This functionality is provided by salesforce for developing custom salesforce applications for sharing.
Master detail relationships are tightly bonded and due to this, parent has all the control access of record sharing whereas child records can only be controlled by parent, having no separate record access hierarchy.
In that case, the object you are trying may be a child of some other object. Please make the parent’s OWD as private then it may show the button.
OWD must have been set to public. As it is public, no sharing is required and hence the button is not visible. Try changing the OWD to Private, then the button may be visible.
OWD may have been set to private. One possibility: No sharing rule is created and the user is the record owner. In this case, only owner can access his owned records. Another possibility: Sharing rule is defined for sharing record access to a particular user.
Public groups, users or roles.
Admin can use public groups. Public groups generally consist of a number of users in them so if a record is shared with a particular public group, that record will automatically be shared with all the members of that group.
Sharing rule will be ineffective as OWD is set to public. Hence all leads will be visible to each and every user.