Once your Flex account is activated you cannot move into another family. However, you can form your own family and invite others to join. Please note that those joining your family have to be new Flex subscribers.
No, unfortunately you will not be able to join an existing family once you’ve created your own Ultra Flex plan. However, you can add more lines to your Ultra Flex for Individuals plan to turn it into an Ultra Flex for the Family plan anytime.