When viewing the Channels section of the app, click on the Plus button to the right of "Channels":

This will bring up a menu where you can enter the Name, Topic, and add Members to the new channel. 

To add members to the channel, simply type in the first few characters of the user's name or email and click on the "Add" button to the right of their name:

Click on the Create button at the bottom right hand of the window to create the new Channel: