Easiest way to have an RSS feed with group contributions

George, you need to check if the publisher's feed is pushing its RSS feed with some push protocol; PubSubHubbub for example.

PubSubHubbub is an open protocol for distributed Publish/subscribe communication on the internet. The protocol extends the Atom and RSS protocols for data feeds. The main purpose is to provide near-instant notifications of change updates, which would improve on the typical situation where a client periodically polls the feed server at some arbitrary interval.

Here an hub hosted on Google App Engine. Give it a try.

You could also register to Superfeedr that handles pushed contents via PubSubHubbub, RSSCloud, XML-RPC or SUP.
Answering some comments, if your feed's publisher does not use this kind of push technology, superfeedr (via polling) guarantee delivery of new entries within 15 minutes of their publication.

if you can't find the RSS you want on any website, you can always create a custom one yourself with dapper. it's pretty easy to figure out how with their instructions.