You can do everything execpt limit the number of E-mail addresses that can be created. You can create a webid for your friend and give him/her access to certin parts of your panel - and only for certin domains. But if this person can create E-mail address - they'll be able to create as many as your plan allows.
There's two main places to find information. This forum is a good place - and a great search feature. Also check out the Wiki as there's a lot of good informaiton there too. If you don't find what you're looking for feel free to ask here - it's what the forum is for.