The pruning groups are 1) early flowering clematis, mostly species and small flowered varieties 2) mid-season flowerers, including most of the big flowered ones 3) late season flowerers.

So you can deduce the pruning group by the time of flowering - so yours would be group 1. As Janet says, you can get away without pruning.

Even if you don't prune, the plants will flower. It seems to me that the main reason for pruning is to keep the flowers somewhere where you can see them rather than on the tops of the trees.