This can be accomplished either via the WYSIWYG, or using Luminate Online's Standard notation.
- Click on the "If" button in the toolbar
- For group choose "Groups"
- Choose either "Is in group" or "Is not in group"
- Search for the group in question and click on "Select", then click on "Insert"
Now you will see the word "If" and three tabs. Whatever content you put in the tab labeled "Member of Group: xxxxx" with show only to people who are in that group, and then people who are not in that group will see content in the "Not a Member of Group: xxxxx". The last tab, "unknown", will show to users who are not logged in (when applicable).
[[?[[S45:1234]]::TRUE::Message for if IN group ID 1234::Message for if NOT in group ID 1234]]