To have a manager see his subordinates in Identity Portal, you can create a Search where the filter is based on "Logged-in user attribute", as follows:
All of the following are true
Logged-in user attribute
User's Manager Equals Logged-in user attribute's UserId
This will allow the manager to search for users and the only results will be his subordinates. If you wish to have the results displayed automatically you'd add a similar condition on the Predefined tab of the Search. For more details on Searches, see the IP documentation at:
Also, there is a Subordinate Search already defined in the OOTB Team Management Module, so you can also refer to this module as a guide.