You can access the list of participants in a conversation by looking at the details of the conversation, accessible from the "i" in the upper right corner of the screen.
From the list of participants, it is possible to consult the profile of each of them, and even to write them personally.
If you are the administrator of a conversation, you can name other administrators, remove the administrator privilege, and even remove a participant from the conversation. Finally, you can add as many participants as you want to your group conversation, up to a maximum of 150.
Important note: To be an administrator of a conversation, you must either be the initiator of the conversation, or have been appointed administrator by another administrator.
The following videos show how to view the list of participants and add new ones.
Another important note: A participant added during a conversation will have access to all the messages sent in this conversation.