To allow a merchant to view the "Affiliates in campaign" tab in their private campaign, you (the network owner) must enable the read permission for User in commission group in the merchant's assigned role.
You can configure this in the Tools > Roles window. If necessary, create a copy of the Default network merchant role by clicking the icon next to it. After creating the new role, select it, scroll down to find User in commission group, and check the read option. Once this is completed, update the merchant's role in the Tools > Merchants window. Edit the merchant for whom you wish to enable this permission, select the new role you created in the Role field, and save the changes.
The merchant will now be able to see the affiliates added to their private campaigns.
If you would like the merchant to be able to approve new affiliates in Public - with manual approval campaigns, please also enable the write permission for User in commission group. This will allow the merchant to change the status of affiliates in the "Affiliates in campaign" tab.