How do I add people to a channel?
1. Start from your teams and channels list.
2. Click the more icon next to the team name, and then click Add members.
Another option:
1. Click View team
2. Add people from the Members tab.
3. Once someone has been added to a team, they'll automatically have access to all that team's channels.
Recommendation: When you are finished adding people to a team, @mention them in the new channel so they know that it exists.