To add further approvers to an Approval Group:
Go to: Settings
Then Approval
From here Select the Approval Group.
You can add the approvers to which ever stage necessary.
Please also make sure that the user is assigned to the approval group you are changing.
Open Settings
Then go to Users,
Then click Edit User.
Here you can check the approval group assigned to them.
Note: newly added approvers will only see claims submitted after they were added to the approval group. If they need to approve existing claims then they will need to have permission to approve on behalf of an existing member.