There's no reason a small group can't scan in then wait just beyond the inner most tapstile for the last person to finish. Other than to send a message to that person that they are a member of your family up until, but no further than, they cause you a 2 minute delay entering WDW.
Well, it's kinda both. Disney hasn't published who makes their scanners but a security tech journal I read gives it up as most likely a Lumigdem Venus (
https://www.bayometric.com/fingerprint-scanner-lumidigm-voyager-v311-v31x-sensor/)
The scanner takes a multispectral pic of your finger and generates a 500 dpi image it then runs a vision routine that looks at all the lines on the print and identifies lines that split into two lines, called bifurcations. It records where on the image all of these bifurcations occur into a unique file attached to your ticket number and then it dumps the image. The fingerprint data in this file, called minutia, is the only thing that's saved and it can not be reassembled into an image of your fingerprint. It just can't, it's only the individual points on your finger where one line splits.