I don't know of any public/gov't run transportation that runs up I-70. Mostly reasonable privately run van services run up there though. Such as Colorado Mountain Express (CME) has about a zillion 16 passenger vans on I-70 going each way to the various resorts it always seems (OK, slight exaggeration).
So not a literal private van for you, but with a small group of folks arriving/departing at roughly the same time...