The Snowbell is a slow growing tree, one to be truly appreciated for its classy presentation and dainty white flowers, so you would want something not to overpower or be in bloom at the same time. I'd suggest pairing it with the Queen Anne agapanthus. The snowbell blooms in the spring, the agapanthus in the summer, giving your planting bed color at two different times of the year. The agapanthus stays low (not counting the blooms, foliage clumps to only 18 to 24 inches) and is quite effective when mass planted, something you could group around the tree or along the edge of the bed itself.