Many-to-many relationship when one entity is without attributes
The 'Right' thing to do depends.

If there's any chance the cluster might end up having attributes in the future, then yes you should have a join table.

Otherwise, if you are trying hard to normalize your data, then you should have a join table, if you are more concerned about minor size/performance boosts then you could skip the extra table. Personally, since I don't have a time machine to see the future, I'd make the extra table anyway.