steam-join-all-profiles-groups
This is a script for mass joining Steam groups, by loading a users profile and automatically joining all of the groups they're in.
How to use
Install one of these extensions for your browser:
Chrome: Tampermonkey
Firefox: Greasemonkey
Opera: Violentmonkey
Go here and click "Raw".
If you are on chrome, it should give you the option to install into tampermonkey.
When you have the script installed, just load up someone's steam profile, and wait.
You will slowly start joining all of their groups.
(You may need to refresh the page for it to join more.)