I'd check what your permissions are as a HubSpot User.
They've recently redone how permissions are done, there's much more modular control. But anyway I'd imagine you're just a user and not a "Super User" which has permissions for everything, including adding users.
You'll run into that every so often as a user, you need permissions to do certain actions such as adding users or creating bots. You should be able to see everyone's role in your account section and the Super User should have no problem adding someone.
