how to do array macthing and printing?

I am creating a contact app so I want to make groups of contacts like WhatsApp group.

if we open the group I can see the contacts and create a button that can add new contacts.

if I clicked the button to add a new member it show all contacts in my contact list

HOW CAN I MAKE IT SHOW ONLY THE CONTACTS THAT ARE NOT ALREADY IN THIS GROUP?

I am using PHP to create this