To elaborate I should mention that my understanding of the catacombs is that they are a series of tunnels underneath certain portions of Paris. Is makes sense to me that if this is the case and there are multiple entrances that any form of French Resistance might use them to their advantage. Was this the case?
The Germans used a part of the tunnels as a transmission HQ, under the Lycee Montaigne. It's still open today (in the illegal part however). They entered through staircases, that are close to the HQ.
The Resistance is said to have installed a secret base in a preventive bomb shelter, under Place Denfert Rochereau. This is actually very close to the official visit circuit by the way. It's also right next to the IGC (Institut General des Carrieres) HQ
At the time, the catacombs were open everywhere (stairs and manholes) so it's very certain that many people used them to hide food or weapons, or to smuggle goods for black market. The tunnels are a total of 300 km so it's pretty easy to hide and smuggle in such a context, and can explain that Germans and Resistance never ran into each other.
You may want to try reposting this to the CoD AMA we have going, as there are some folks who can talk about occupied PAris there.