bbs.spamgourmet.com

I want to use the trusted senders to receive messages from some Yahoo Groups I belong to.

These arrive with:-
From: [original author of message]
Sender: HCG1@yahoogroups.com

Is it possible to use the Sender: URL?

I tried, but these still seem to increment the message count.

you should be able to add that sender address (or, more specifically, the To: address) as an *exclusive* sender in the specific address record. Exclusive senders (unlike trusted senders) match against the To: address specifically for situations like this.