To get this functionality you're going to need to modify the existing DB structure and the corresponding object structures that are used to maintain the communication with the database. You could create a user_tenant_mapping table that would check that a user is tied to the data being pulled from the database, or just add a tenant_id column to the act_id_users/act_id_groups table. Both of those have their own difficulties, but could function to do the job you're looking for. You could also extract out the tenant information into its own table and correlate the IDs to the data and the users.
The difficult thing here is that Activiti Community doesn't support any OOTB methods to enforce multi-tenancy rules; it just provides the ability to 'partition' data into different tenants, but the ability to implement privacy if necessary is on the developer.
I know it's not the exact thing you're looking for, but in the same thread we were discussing yesterday, the 'Multi-Schema Multi-Tenancy' option would allow for complete separation of tenant data with much less additional workload overhead, and you wouldn't need multiple Activiti Engines to complete it.
Hope this helps,
NOTE: As Ratik has started a new thread, this previous thread is might be relevant.