Presumably you've made these accounts yourself from the control panel at:
If you've made the dns records for your site when you visit it you should get the default page come up. Looks a little like this one:
I assume you're trying to get a client to be a reseller or have access to the control panel to add more of their own sites etc yes?
To add a reseller you'll find that on the clients tab, then 'add reseller'.
If you want someone to have access to the control panel like you do, it's the system tab, then 'add user'.
All they then do is log in the same way you do:
with the details you set up. They will have access to the modules that you gave access to.
First things first though is to test out a default site that you set up. Remember unless you have set up dns records for your domain no one will be able to find it on your server.
I don't want to go over stuff you already know but just in case...
*Set up a domain with a registrar which will be used for the name server on your setup. Tell the registrar to set up a glue record. This points that domain to your ip address.
*The on your server set up the domain that has the glue record to act as the name server domain on your server, so you can make ns1.yourdomain.com and ns2.yourdomain.com. The dns wizard virtually does this for you.
*Then you need to add the domain that you intend to host into your own dns records using your ns1 and ns2..
*Then you need to go back to your registrar for the domain your are hosting and change the name server records to your own ns1 and 2.
So when someone looks for your domain from the internet and put in their browser yourhostedcomain.com it looks for the .com name servers who say they don't know where it's held but they do know someone who may, your registrar, your registrar (or duplicate record server) says I don't know where that record is held but i do know it's held on ns1.yourdomain.com AND i know the IP of that name server, so it send the user using the IP to ns1.yourdomain.com. ns1.yourdomain.com will reply - yes, i know where that domain is held and because I'm a clever ISPConfig3 server and I do the lot, it's held on me so look for your site at my IP address.
That's my interpretation of the DNS model and I can't guarantee it's completely accurate but i think it's about right - and that's the bit I think you're missing... unless i've made a grave mistake and you can infact see your default sites from other internet machines?
The other way i believe is to make DNS records with your registrar if they offer DNS services. I think you'd have to make an 'a' record with them initially pointing to your IP... but that's not nearly so much fun.
The schlundtech guide written by falko as attached above is a good one to follow plus it's well supported as it came from this site. However, if you have difficulty with the German language on their site you can apply the same principles to other registrars. As well as schlundtech I've used UKreg which offer glue records.
I think it helps to understand the DNS model to know how to implement it yourself.
I'm not an expert on this matter at all and only speak from experience so anyone feel free to correct what I have written if I've got the concept or models wrong.