They are mainly an airport shuttle, I once asked a similar question and was told you might be able to go from one location to another without going through DIA, but they didn't think so. With the recent change to Groome, you might call the main number to see if there is a change in policy.
Im really sorry, but the last time I checked on a similar situation the answer was no. On the other hand, they have just recently changed owner/management. Who knows what they might offer now. It's definitely worth a phone call.
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