We have enabled customer communities, I want to incorporate a link to the community for the community user, when he receives an email, as my system admin is creating a case, then we use workflow to send the email to the contact of the case who is the community user.(as the user can only login to the external communities)
The easy solution will be create a Custom setting and store the community URL there. Now refer that custom setting in your email template.
If you are using VF email template then include a component in that and in component controller you can easily get the community URL.