It does appear to be a Hydrangea but it will be difficult to determine which variety without a photo of the bloom. It's completely up to you as to whether to grow it as a mass or as a tree. Regarding pruning, wait until after flowering unless you don't care about flowers.
If you only care about getting it to look more like a tree, remove all branches except the straightest, most upright one. This will become your trunk. You'll need to pinch back any new growth that appears on the new trunk during the remainder of the season. As soon as this new trunk is as tall as you want it, allow side branches to grow near the top but continue to pinch out any other new growth on the trunk. Support of some kind for the trunk may be necessary when if finally does flower as the flowers are very heavy.

Thank you for the advice! As this plant has been such a poor bloomer, I'm going to begin pruning it this season before it gets any larger. Hopefully, as it continues to grow the trunk will thicken and that will help support the flowers it may have next year. I always have to amend the soil here, but this plant has been at the edge of a fairly woodsy area and pretty much on it's own... I haven't done any soil conditioning at all. I'm looking forward to seeing what happens with this guy next year!