On the desktop/web app, you can hit the + button on the bottom right portion of the Conversations page/Conversation list, then insert as many contacts as you want into the compose field. After selecting a contact, their name will be populated above in a chip. You can continue to insert as many names as you want. If you don't have a contact saved, you can type their phone number into the contact entry at the end, and it will be added to the contact list when the message is sent.