⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Steam removed the option to invite multiple friends to your Steam group.
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀This guide shows you a method to still do it.

⠀⠀⠀⠀⠀⠀⠀⠀⠀First you'll need your group ID. To get the ID, visit your group in a browser.
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ Paste the text below after your group URL and visit the new link.

/memberslistxml/?xml=1

⠀⠀⠀⠀⠀⠀⠀⠀⠀ The second line of text should show you the group ID. You'll need it later.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Open a new tab in your browser and visit your friends list.

steamoss.com/my/friends

⠀⠀⠀⠀⠀⠀⠀⠀⠀Right click the page, go to inspect elements, and click on the console tab.
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀You'll want to paste the code below into the console

const groupID = 'GROUPID';
const friends = GetCheckedAccounts('#search_results>.selectable');

InviteUserToGroup(null, groupID, friends);

⠀⠀ Before you send this code, go back into your first tab and copy your group ID that we found
⠀⠀⠀⠀ in the previous step. You'll want to paste that number in the code where "GROUPID" is.





⠀⠀⠀⠀ Once the code is modified, simply press enter to send the code. This will send a group
⠀⠀⠀⠀ invite to all your friends. You may get a failure message, but ignore this. It only means
⠀⠀⠀⠀⠀⠀⠀⠀⠀ that friends who are already in your group won't receive a second invite.