1) If you are managing the list for the group - then go with an LDAP server - let me know if you are going to share the contacts 8)
2) The most flexible format is likely vCard as that can be consumed by the most different clients/services. Sharing would also be the easiest with a web server, a carddav server or any of the file server services (many of which are free - if your contacts are public). Like Github.
3) The most modern would use the semantic web - create FOAF files for your contacts (much like vCard) and either present them like files or set up an RDF store and publish a SPARQL endpoint. If you run KDE, you already are running Virtuoso for Nepomuk - others include Jena, Sesame (openrdf) and 4store among many others.
If these are PLUG contacts, maybe the website can work with a LDAP backend - I don't know if having a roster complies with PLUGs anarchistic culture.....