It’s nice that you can host your own Contacts server with this app on NextCloud, but there are many people out there such as me, who already have a CardDAV server and just want to use the Contacts App within NextCloud to manage them.
Two questions:
Greetings,
RastaTux
The Contacts app is only working with the Nextcloud CardDAV server. If you already have a CardDAV server, you can move its content to Nextcloud or try to sync both servers outside of Nextcloud. For Linux the Cadaver command line tool might be helpful to setup such a job.
The Contacts app only supports groups in the “CATOGORIES:” field yet, but no VCard groups. A feature request to add this function already exists on Github.
Okay, but the thing is i don’t want to move my contacts over… I know it’s not easy to integrate, but maybe syncing to another server would be a nice feature, too? 
As far as I remember this has already been discussed in the past and declined, because it has not been identified as a core function of Nextcloud. As written you can use external tools, outside of Nextcloud, to sync your contacts with another Carddav server.
For me, using an external tool is not optimal as it just raises the potential for problems and adds complexity. Let’s be honest to ourselves: one day - when I need them the most - I will notice that the sync failed for any reason and I have two cardDAV servers with nonconsistent contacts. 
Still thank you for your help, j-ed! I consider moving my contacts over as it’s the simplest solution.
I agree, keeping the same data on different servers could cause unexpected data inconsistency.
I have a working setup of vdirsyncer and two servers. One server is a DAViCal server that has been in production for many years. Using vdirsyncer I sync it to/from a local vdir.
This local vdir is also synced to/from Nextcloud.
Some clients communicate with the DAViCal server, others with Nextcloud.
Important (for me): the entries in the local vdir are stored in a git repository, making the risks of losing information very small.
This setup has been stable and robust for a while now.
Okay. But took a while to set up i guess, especially if you are no experienced administrator. Just syncing your contacts with an external server from nextcloud would be way easier for this usecase.
Unfortunately Nexcloud doesn’t allow its Calendar and Contact apps to work with an external server, so that’s where it ends.
If you are looking for an alternative good, browser based calendar/contacts client: take a look at http://www.inf-it.com/open-source/clients/infcloud/ (development is currently stalled but will resume soon). It’s a bit slow, though.
As i said, i hoped that this could get into a feature request, because im sure im not the only one with this problem. 
Thanks!
Hello!
I can already tell you that it will not 
We have already a deep dav integration (cal/card) into nextcloud as it is used by a lot of others apps/services within it. It would make no sense for us to start implementing such change as it sould just be a very tiny niche of people who would know what to do with it and/or have a real use for it, but it will also complicate our work a lot more.
Thanks a lot for your suggestion, I can only suggest you to import your carddav content into nextcloud and benefit from the other related stuff (contacts menu, contacts app, calendar suggestion, mail contacts suggestions…) that comes with it
Cheers,
John
Oh, I´m one of the special users who need it really.
Sciurius, are willing to help me out with the configuration of VdirSyncer?
I´m not a programmer but a little generally knowhow in IT.