Private chats for friends.
Already joined on another device? Open Settings → Link a new device there, then enter your number and the code it shows.
See which of your contacts are already here. Their numbers are only used to look for matches and are never stored.
iPhone: in the Contacts app, open a contact → Share Contact to save a .vcf, or export all contacts from iCloud.com → Contacts → select all → Export vCard. The file is read on your device; only phone numbers are checked.