How to use a second domain

I have added a second accepted domain name to exchange but am not sure what to do now.

I want to add a second email address to a current user so that they can access it on there phone and through outlook\owa

I also wish to set up 2 other users that will have access to just email if possible as the second domain is for a side business and not related to the first.

I hope you guys can make sense of that as any help would be great. Sorry if this is a simple thing I looked on google but I was not really sure what I am looking for.