You have to open the link and copy your steam ID instead of "multiplay".
Press enter to reload the site and you will get a bunch of numbers. You can also watch the complete tutorial on youtube.

Open Steam in your browser and go to friendlist.
If you are on your friendlist press f12 or Q to open the console.
Copy into your console this command:

var groupID = 'idhere';
var friends = GetCheckedAccounts('#search_results>.selectable');


InviteUserToGroup(null, groupID, friends);


Copy your group ID (numbers) instead of idhere.
YOU HAVE TO LET THOSE ' ON THE SITES OF THE NUMBERS!!

After this you just have to press enter.

Done buddy :-)