There may be a possibility to customize the group - members list widget if there is the ability to pull back a group member join date. It has some sort options in it already, so may be something like adding in another filter option.
We also have our site - newest members widget, which I know pulls back when a user joins the site, so that could be something to look at to modify as well.
As Brian mentions though, no out of the box widget today to accomplish that currently.
