I have 'Princess Diana' up into a Rhododendron, and it suits it well. In another garden I have seen 'Sizaia Ptitsa'meandering around. I was big and very beautiful.

Any of the herbaceous, non-vining types of clematis can be used as a groundcover - integrifolia and its hybids, heracleifolia, etc. If the plant likes to vine, it will have a tendency to grow upward, so you can expect it to clamber up shrubs and perennials in the area. Be careful on mixing vigorous vining types with smaller plants as they can sometimes overwhelm them.

Here is a link that might be useful: herbaceous clematis