I need this very thing and was asking HubSpot customer service about it just this week. They told me to post here. It seems like that should be a fairly easy option for them to provide, so I hope they make it available!
Upvoted. My reason is because we have customer 'hubspot user' properties and currently we can only select one user from this property. I need to be able, on certain deals, to select two or more because two or more sales reps have been involved on the deal.