Are you charging them rent? The amount they help pay on utilities would be added to the rent amount. If they are not paying rent I would think that technically speaking, their share of the bills counts as rent:
Expenses paid by tenant occur if your tenant pays any of your expenses. You must include them in your rental income. You can deduct the expenses if they are deductible rental expenses. For example, your tenant pays the water and sewage bill for your rental property and deducts it from the normal rent payment. Under the terms of the lease, your tenant does not have to pay this bill. Include the utility bill paid by the tenant and any amount received as a rent payment in your rental income. http://www.irs.gov/businesses/small/article/0,,id=238662,00.html