If you are on the Advanced plan or higher, you add them just like any other user! Click on People then on Users. From there, click the blue Add User button on the top right. Fill in their information and change the Role from Learner to Manager. Once that’s done, click Create New User on the bottom.
You can also add Managers directly to a group by clicking on the Group, navigating to the Managers tab, then clicking + Create New Manager.
Alternatively, you can assign an existing user as a Manager. To do this, just click on People then on Users, and click the Edit button beside the user you want to promote to a Manager. Next, just use the drop-down menu to change their Role from Learner to Manager. You can also use this field to change a Manager back to a Learner.