To add a user to a chat channel, the user needs to be a Team member. Only then can you add the user to the chat channel.
Add a user to a Team
Click on the action menu (three dots to the right of the Team name) and select Add member...
Find the person you want to add and Save. The user is in the Team, and we can add the person to the chat channel.
Select the channel you want to add the user to. Click on the action menu and then select Add team member...
Add the person the channel and channel members see that Filip has joined the channel