It depends for how many users you want to create it. Coding a group chat for
up to 20 users is quite easy, all you need to know is just basics of HTML, PHP
and SQL. You might be interested in those courses:
You can just create a page with a form to submit a message. All your users
will visit this page and if you add some JavaScript timer or AJAX to refresh the
page every few seconds, it'll look and feel like a live chat.
If you want to make something bigger for more users, refreshing the page
every few seconds would fry the server. You'd have to use websockets and create
a custom server, which is quite complicated.