Hi Roger,
with the enterprise edition, this works by binding an LDAP group to the channel, so as soon as a new user with a matching group membership is being created in your mattermost instance, this user will automatically be added to the channel which is linked to this specific group(s).
Without this feature you would have to automate that using the API or mmctl f.ex. Base on what criteria would you want to add people to channels? Are these private channels you want to add them to or public channels? For the latter I think you could also use the welcomebot-plugin.
Best,
Alex