You can send messages to groups of people, but can’t add or remove people from the conversation once it’s started. 

To start a group conversation with multiple people:

You can now type your message to all people in the group, and send files as required. If you share a file from Google Drive, everyone included in the group conversation will automatically be given view and comment access to the file. 

If you’ve been included in a group message, you can leave the group if you wish to hide it from the left-hand side of the screen. To do this, position the mouse over the conversation, click on the ellipses, then choose Leave

Click on Leave Space to confirm you want to close the conversation and no longer receive notifications of any new messages in the conversation.