Manage channel members
Add members to a channel
Any member of a channel can add new members to a channel. Select the channel name at the top of the center pane to access the drop-down menu, then select Add Members. Choose Add next to a user’s name. Users already added to the channel will not appear in this list.

You can also add users to channels within their profile pop-over by choosing Add to a Channel and selecting the channel you want them to join.
