Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.

It is not compulsory for a man wishing to get married to a second wife to seek permission or even to reveal it to his first wife. There is no problem to go with him in hajj after being married. He just makes it a secret to his wife, but  according to authorities and registration processes, you are his wife and he is your mahram in this case.

 

Almighty Allah knows best.