Enable user registration - eJabberd
You have two options regarding user registration. You can either manually create user accounts on the server, or you can enable in-band registration, which allows anyone who can connect to the server to register an account.
If you want to create accounts manually, run this on the server for each user:
sudo ejabberdctl register <username> <domain> <password>
If you want to enable in-band registration, find the following line in /etc/ejabberd/ejabberd.cfg and change the deny to allow so that it looks like this.
{access, register, [{allow, all}]}.