Have you considered using Business Rule based assignment for your tasks?
i.e. On the Assignment tab of your HT use a BR to assign to a group based on a list of companies in a decision table.
Your Workspace users could then be added to the applicable groups.
Certainly other ways to achieve this so it depends on the specifics of your solution.
Thanks for your reply..
Yes. The problem is all these users are belongs to same group but they should not see each others tasks.
i.e. Managing Directors of different subsidiaries in one organization.
They should not see each others tasks, but they are having same interface. Having separate groups for each SBU is impossible since we are having more than 50 subsidiaries.
As was suggested above, you might want to consider a business rule to do this. Given the subsidiary's name as a parameter this blog post steps you through it: http://www.avioconsulting.com/blog/creating-parametric-roles-using-business-rules. In your case, you'd assign the task to the user id of the person who is the managing director (the blog post instead has it assigned to an LDAP group).