Steam Group Mover
Configure the two variable (var) tags below with the correct CustomURLs for the groups you'd like to move to/from.
i.e. http://steamcommunity.com/groups/[CustomURL] << Your CustomURL is the text after /groups/.
After configuring, save the script and then visit the group page of the group you're moving to (output_group_CustomURL).
The script will then execute after the page has fully loaded, and logs successes/failures to the JavaScript Console.
It takes awhile to completely execute so being patient and viewing the console is a must.
After the script finishes you will need to manually turn off this script or it will run every time you visit the group.
Invites are limited to ~300 invites every ~3 hours, so this script must be run more than once for larger groups.