Access the search bar, type a letter or number, you would see suggestions based on what you type, click all

Simply type @ then type the members name

Only channel admins can update channel name and description, access your channel, click on the three dots and click admin

As a member of a private channel you cannot invite a member. As a member of a public channel you

On Web, scroll to channels on tea spot, click Channels, you would see the channels you belong to On mobile,

On web, scroll down on tea spot, till you find Channels, click join channels, you would see all the public