Hi there, its a cool idea and as per your brief, Ive tried to incorporate your requirements into the design. In the Login screen, you can see ive given placeholder for the intro text, showcase benefits in horizontal scroll format, login button will will push the benefits part up for user name and password fields. In the inside screen, the user will see 4 recent chats and this section can be made scrollable vertically, or there's a view more recent chats button given to expand the list. Below that is the groups you're part of which can be scrollable horizontally. Below that we can add a "featured groups' based on your interests etc.
PLease let me know your thoughts.