Your "guests" might become unintended subtenants who are entitled to notice before eviction
Q: If you are the only one on a lease, do you have the right to kick out your friend, boyfriend, or girlfriend without notice?
A: If you allow someone to live with you, that person may have rights as a subtenant. You may have to give them a 30-day notice and file an unlawful detainer action to evict them.
A “guest" might become an unintended subtenant with the right to notice prior to eviction
A tenancy at will—although consensual in nature—is not based on an express rental agreement. It’s created when a tenant takes possession of the premises with the landlord’s permission, but for no stated term and without payment of rent. (Covina Manor, Inc. v. Hatch (1955) 133 CA2d Supp. 790, 793.) Upon agreeing to pay rent to the tenant, a roommate may become a subtenant on a month-to-month basis. (Valley Investments, L.P. v. BancAmerica Comm’l Corp. (2001) 88 CA4th 816, 823.) “At will" tenants cannot be evicted instantaneously. The landlord’s termination must be preceded by 30 days’ written notice. (CC § 789.) But it appears that no notice is required by tenants who seek to terminate a tenancy at will. (See, Miller & Desatnik Management Co., Inc. v. Bullock (1990) 221 CA3d Supp. 13, 18.) The subtenant’s rights are against the tenant-sublessor. A subtenant who defaults in rent due under the sublease may be evicted by the tenant in an unlawful detainer.
Rent control laws may protect a person not on the lease
Many rent control laws extend their rent control and eviction control protections to lawful occupants—whether or not those occupants have signed rental agreements. (See Santa Monica Rent Control Charter Amend. § 1801(g) & (i).) Under such an ordinance, a “roommate" may have the rights of a “tenant" where the roommate’s occupancy was approved by the landlord—expressly or impliedly. (DeZerega v. Meggs, supra, at 42.) In DeZerega, a roommate whose occupancy was expressly approved and authorized by the landlord was entitled to protection under the Berkeley eviction control ordinance even though the tenant named in the lease vacated. It didn't matter that the tenant’s lease prohibited assignment or subletting. Even assuming the roommate could be viewed as a subtenant, the lease expressly authorized “roommates" and the ordinance expressly extended its tenant protections to “any other person entitled to the use or occupancy" of the rental unit.
A lease prohibiting roommates may not be enforceable
If the lease requires landlord consent for a sublease, and the withholding of consent is subject to an express or implied “reasonableness" standard, tenants who feel that consent is being withheld unreasonably may “take their chances" and enter into the proposed sublease despite the landlord’s objection. If the landlord brings an unlawful detainer after the tenant transfers possession, the tenant may then defend the suit on the ground that the sublease was not unauthorized because the landlord acted unreasonably in withholding consent. But the tenant bears the burden of proving the landlord acted unreasonably. (CC § 1995.260; Kendall v. Ernest Pestana, Inc. (1985) 40 C3d 488.)
If you don't want to go through the hassle of giving someone 30-days notice and filing an unlawful detainer action to evict them, think twice before you agree to someone living with you. The fact that you are the only one on the lease and the other person is not paying rent is not determinative.