I have searched for solution and find out that the contact with multiple companies can able to access the companies in which it is assosiated with only with different username or with different password with same login credentials he/she can not access. So the netsuite user has to provide different user name or password while giving access to the contact.
That will notify the contact with a link where he can provide email and password.
With the below screenshot tells how to provide access for the contact and also we can come to conclusion that with the same login credentials the contact can’t access for different companies.
With different customers and same contact accessing the different company account.
Customer 1: For this customer i have added an item to the cart.
Contact: Can able to access the customer 1’s account with the below credentials.
Same item is displaying in the cart.
Customer 2: For this customer i have added two items to the cart.
Contact: Was able to access the customer 2’s account with the below credentials.
So conclusion with same login credentials the contact can not access different companies in which is is assosiated. So he needed to be provided with different user name or password for different companies.