You can easily turn a Direct Message conversation into a Named Conversation. This will allow you to add or remove members from the conversation, preserve the conversation for new people added to catch-up, and have more access control options.

1. Go to a DM and tap the “...” in the upper right corner to get to “Conversation Settings”.
2. Tap on “Members”.
3. Tap “Convert DM into Named Conversation”.
4. Name your conversation and tap “Convert”.