If you're the landlord, and are authorizing the sublease, then the sublease is between your tenant and their subtenant. If you choose to sign documents with the subtenant, you run the risk of making yourself a landlord -- nobody can comment on the legal ramifications of theoretical documents.
You can find a wide range of forms that various landlords use with tenants, in relation to authorizing subleases, through your favorite search engine. (Here are some through Google.)

