To add a new collaborator to your Workspace, navigate to the Workspaces option on the left nav pane and select the Workspace to which you are adding a collaborator. Once opened, you should see the icon to add a new collaborator at the top right of the Workspace.
Click on the button with the person icon and plus sign, and this will open a window where you can add a new collaborator to your workspace. Enter the email address of the individual you are adding as a collaborator, and select "Send". From here you can also view current collaborators on this Workspace.