So you’re saying you never read the lease you signed?

You simply signed an addendum stating you become a co obliger to whatever lease was in force at the time?

what was your written or verbal agreement as to the amount you would be responsible for each month?

you need to understand that unless a lease apportions the rent among the tenants, each tenant is liable for the entire amount. The lease does not specIfy what any partOcular tenant owes each month. It is by mutual agreement between the co tenants that each person rent is established