This should work, but really only applies if it is simply off-screen. You could 'slightly' mod whatever group window you use to force it on-screen.
Put something like
OnShow="Location=0,100" in the first <Page ...> tag of your eq2ui_mainhud_groupmembers.xml and save it to your mod directory. If you're unsure about mods, refer to the
Installation FAQs.
When you log in, move the group window, it's location should save. Then remove that line so it doesn't reset the next time you log in.
Maybe there's an easier way, anyone?