I have a lookup on Account, this is a lookup to a Account which related to Contacts who are Community Users. I want to create a filter for Account lookup so that it only show the related to Contacts who are Community Users. Can anybody guide me how to get this solved.
What you're asking is going to be challenging to do unless you've created a RecordType or a special field in Contacts for the Community User to indicate that the Contact has been enabled for the Community. If you haven't, you'll need to collect a set or list of all the contacts for the Account, then run a query on Users (I'd limit it to those with the PortalUser profile) to see which ones are returned who have those contact ID's.
Its not going to be a single look-up process that involves a single query without adding additional fields to contacts.