However, here are a few options you could use, depending on the tone you want to convey:
Formal:
* A collection of matchbox collectors
* A group of matchbox enthusiasts
* A society of matchbox aficionados
Informal:
* A bunch of matchbox nuts
* A pack of matchbox hoarders
* A crew of matchbox fanatics
Ultimately, the best collective noun for matchbox collectors depends on the context and your personal preference.