Welcome to the Asana Community Forum @Yashvi_Sanghvi 
If you have an Asana organization and you are adding other members that have an email with the company URL they will automatically be considered as part of the org meaning added as paid member.
There are ways to do it by having independent Divisions or Teams that are upgraded on separate paid plans.
In organizations guests would only be people that you invite with an email not from your domain. Guests would also only have access to certain features.
In order to discuss specific options such as divisions or only upgrade for one of your teams it is best to reach out to support and they will be able to assist.