I'm signing a lease for a rent-stabilized apt which I want to share with roommates. My question is: how much is reasonable to ask them to cover: their "share" or more of the rent? As the lease holder is it taking advantage to have my roomies pay more?
A lot of factors could go into how much you charge roommates who aren't on the lease. For instance, do they equally share space with you? Is your bedroom significantly larger? Do they have the same access to common areas? Or, is a potential roommate a clear fit for you and the apartment, except for the fact that he or she doesn't have enough money to come up with an equal share? There are lots of things to consider.
An easy solution is to just have the new roommates added to the lease, if your landlord will let you. Then, you all have the same rights and you can divide everything evenly. If this is not an option, though, then consider things like space and make your decision accordingly. Because it isn't fair to charge someone equal rent if you aren't dividing everything in the apartment equally. (Just like it wouldn't be fair to charge a roommate half of the cable bill if she never watches TV, or to divide the phone bill evenly if you only ever make phone calls on your cell phone.)
If everything is being divided equally, though, then you might even pay more yourself, because you have the privilege of having your name on the lease which affords you many more rights as a tenant. (Note: tenants and subletters who are not on a lease have virtually no rights.) Otherwise, equal payments would be a good solution. It is unreasonable to ask your roommates to pay more, unless one of the roommates is getting much more space or a more desirable bedroom or the better view, or what have you.
Whatever you decide to do, you should be upfront and discuss these things with potential roommates, and work out a deal that works for everyone involved. And if you do strike an agreement, be sure to get everything in writing (especially if the new roommates are people you didn't know previously). This way, everything is clear up front.