Does your newsletter is scheduled? or is it already been sent? If it is still scheduled to send on a later date and if your sending filter sends subscribers via tag, you can add new subscribers to that tag, and the new subscriber will be included as a recipient as long as the broadcast has not been sent.
I also recommend creating automation that will tag sign-ups so you can label or identify your subscribers. Here is a guide https://help.convertkit.com/en/articles/5523023-how-to-create-your-first-convertkit-visual-automation5
If the broadcast has been sent you need to duplicate it and send it again to the new sign ups. But I do recommend to create a automation instead.