So first off. There will be images at the bottom to give a preview. This will add a column to the Inbox/Outbox pages that shows the number of participants. And then once you hover over the number you will see a list of all the participants.
First go to Admin Home > Themes > Layout Templates > Conversations Page > Conversations Page tab(current tab)
On or around line 22 you will see:
<th class="created-by">Created By</th>
Now add this right below it: <th class="pmparticipants">Participants</th> Now switch to the Conversations List tab(on tab to the right).
Save that. Now to edit the CSS. Go to Admin Home > Themes > Colors & Styles > Style Sheet.