Hello Ralph, and welcome to the forum.
There are a number of people doing the shared office idea, it is generally called "executive offices" or "virtual offices" depending on the services rendered such as conference room, telecom, fax, secretarial, etc. Also, you might take a look at allowing your tenants a separate address, they seem to think it makes them look like a bigger company. Search craigslist's office section, and you should find many similar concepts. As far as utilities go, it sounds like you are set up on a gross lease. You're talking about moving it over to a net lease. Depending on what area you're in, pass through charges go by different names, but the concept is the same. Hypothetically, say your 33,000 SF office building has 3,300 total expenses in utilities a month. Set up your passthrough charges based percentage of total sqaure footage they use. In other words, an 8400 SF tenant would occupy 25.5% of the space, and would pay $841.50 in passthrough charges a month.