Yes, scammers/spammers mass text random numbers. Leaving the group confirms to the sender that they have reached a real person and your number is active. They will then record your number and send you more spam.
It's scammers trying to trick you into giving up personal onfo. Block and delete. If you interact in any way, they know they hit a live number.
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This has happened to me periodically. Spammers/scammers get hold of your phone number and add it to a group chat. You can block the originating number, as well as the recipient numbers. Don't reply to the chat, as that will confirm your number is valid.
If you haven't already done so, add your phone number to the National Do Not Call Registry: https://donotcall.gov/
