To do so you simply add the staff you wish to be approvers into a group, and then assign that group to all of the staff you wish to have the group approve holidays for. Admins and managers are still notified and can approve or reject holidays if they wish.
Here's how you do it...
Firstly, go to your Staff Directory and select the members of staff you wish to be able to approve holidays and click on Add to group
Give a name to the Group and click on Add to Group
Go back to your Staff Directory and choose the member of staff you wish to add the Approvers to and click on their Settings tab. Under staff groups that can approve Alice's holiday you can check the box of the name of the group you want to be able to approve that staff member's holiday
Now, whenever that member of staff requests holiday the people in the group receive a notification to approve or reject their request. Admins and managers still receive holiday request notifications in the usual manner. If you do not wish to receive these notifications, or would like to filter them, then go to your profile and click on the Settings tab. Here you can choose the notifications you would like to receive