You can do this in Tools -> Roles window. You might need to create a copy of the "Default network merchant" role first by clicking the little icon next to it. When you create the role click on it and scroll down for "User in commission group" and check the "read" option there. When you are done you will need to change also the role of the merchant in Tools -> Merchants window by editing the merchant for whom you wish to enable this option and there in the "Role" field select the new role you just created and save the changes.
That is it, the merchant will now be able to see the affiliates added in his private campaigns.
Note: if you should wish to add the option to approve the new affiliates in Public - with manual approval campaigns of the particular merchant then please also check the "write" option next to "User in commission group" and the merchant will be able to change status of the affiliates in "Affiliates in campaign" tab.