The onus is on the applicant to provide clear evidence of adequate accommodation.
The ECO's judgement of adequate accommodation is based on the evidence from the applicant.
The tenancy agreement alone may or may not be sufficient evidence for the ECO, but in order to minimise any risks I strongly suggest you follow the same path as most others here have done and also include a letter/statement from the owner/landlord or agent etc that confirms that there is no objection to an additional resident moving into the accommodation.
It doesn't need to be complicated, just a simple written statement is enough.