This is one of those little things that is easy to overlook. I should know, because I didn’t notice it until Sandra Mahan pointed it out last week (which is when she discovered it). You can display all the members in a Skype for Business group chat, even though the title bar only displays the first three or four names.
In this example, I see that there are 55 participants in the ESC_Point group chat, but I only see three names in the header. What I overlooked was the 55 Participants link:
When I click on that link, the chat window splits into two columns, and all the participants show up in the right-side column:
Obviously, that is really helpful when you’re trying to figure out who is on the chat.