Tenancy agreement signed by one of the co-owners only

Dear All,

First I would like to thank you for providing me with your knowledge and suggestions.

My question is that:

1.  A tenancy agreement has been signed by one of the Co-Owners among the 4 co-owners.  Is the tenancy agreement valid?

2. If not, then who will be at fault and what will be the legal status of the agreement which is nor notarized neither registered?

3.  Who will be at fault? the co-owner who signed after receiving some perks from the tenant or the tenant alone?

Thank you.