Here's a step-by-step walkthrough on how to add someone to a Slack conversation
1. In a DM, click on the user's name at the top.

2. In the pop-up, select "Add people to this conversation."


4. Select the user from the list to add to the conversation.


6. Choose whether you want to include conversation history.


8. If you want to include the history, then click on "confirm".

Congrats! You have successfully added someone to your Slack conversation!