1. create a email user and add the Administrator tick.
There is only one Admin user per domain possible.
If you want to create subdomains for your friends I advice you to create a new page and fill in the subdomain name under Hostname instead of "www"
In this case you can create a Admin user for your friends and they can't login to your web
To access just use www.yourdomain.tld
as address your email account (e.g. web1_admin) as user and the password
2. I would create the following entrys:
yourdomain.tld A yourIP
*.yourdomain.tld CNAME yourdomain.tld
mail.yourdomain.tld MX 10 yourdomain.tld
yourdomain.tld TXT v=spf1 a mx ~all
3. if you have installed ISPConfig following the perfect setup then emails are available as soon as you create a email user.
Address is whatever you want and have in your DNS. (e.g. mail.yourdomain.tld)