To add or invite someone to your Squad you need to be an admin or the creator of the Squad. Then firstly, tap the 'People' icon in the bottom right of your iPhone screen.
For iPad users: Tap the people icon in the left menu.
Tap the icon in the top right corner of your screen next to 'People'.
Enter the individual's email you'd like to invite. You can also import contacts from your phone book by clicking the import tab, and selecting all the users you'd like to invite. Additionally, you will have to set a display name for the user you are inviting, which can be changed later.
Check out Managing Roles and Permissions to read more about each role in squads, pods, and group chats.
After you've sent the invites out that you wanted, you can also manage your pending invites by clicking the Manage Tab where you send invites out. You can also revoke invites by swiping left on the invitee's name.