@varunarathinam unfortunately a dashboard is a kibana object that the Kibana server opaquely saves inside the kibana index. Elasticsearch only provides the storage layer for the Kibana index and does not understand what a Kibana object is. This is why there is no ACL rule for dashboards.
And even if there was, you would need a similar one for all kibana objects: index-patterns, visualizations, short URLs, saved searches, etc.
The only sane way to do this is to associate a different kibana index (we call it tenancy) to user or group. This logic needs a good deal of Kibana client and server side code, and is definitely not trivial to achieve. Our solution to this problem is called ReadonlyREST Enterprise.