There are two ways to assign approvers on group level.
From within the group settings
Go to 'Settings' - 'Company Settings' - 'Groups', then select the group you want to assign an Approver or Controller to. You will see the following field.
You select the Approver/Controller from the dropdown menu and click 'Add user'.
It is possible that the designated Approver / Controller is a member of the group in which he or she approves. If you wish to assign this group-approver a separate approver (so he or she cannot approve their own expenses), you can designate a different approver in the field Assign a different approver for these groups.
Giving approval permissions to a user
To do this you have to go to 'Settings' - 'Company Settings' - 'Users', and select the user you want to make Approver/Controller of a certain group. Under the 'Roles' you can tick either the 'Approver' or 'Controller' box (or both) and select the right group.
In case of Approval conflict you can scroll down to the 'Groups' overview of that user and assign the right Approver/Controller to that person.