Slack has message events for message.group, message.im and message.mpim.
As I understand it, im means it's a private message to one person, and mpim is a private messaging conversation with multiple people. A group is described as a private channel. So what's the difference between an mpim conversation, and a group then?
A private channel is meant to be more persistent: it has a name and a topic like public channels and have no limits.
mpim conversations, on the other hand, are limited to 9 participants.
Adding a member to an mpim will create a new conversation, while inviting a member to a private channel can retain its history.