By default all messages are between two members. But, it's possible to turn it into a group chat by inviting other participants.
Invite other participants
To add other participants you can click the "Add participants" button at the top of the the message thread. Search for and select the name of the member you want to add. The member will receive a notification.
Adding a participant
Unlike in other chat applications, participants are able to read the full message thread. This way the new participant can easily catch up. It's definiately something to be aware of though.
Maximum number of participants
You can add a maximum of 6 participants to a thread.