If multiple departments in your Company are using Asana, and you want to keep your Premium plan exclusively for your team, I would recommend to move your premium plan from the Org level to your team level, this way you will only be charged for your team members. If other department in your Org want their own premium plan, they can subscribe their own Premium Team plan. It is completely ok to have Premium Teams with their own independant plan in one Organisation, in fact it is a common practice in large companies.
If your team “Marketing and Comms” uses 2 teams in Asana (one for Marketing, the other for Comms), you can request a division which will allow you to share one premium plan between two teams!
In all cases, I would highly recommend you reach out to our sales team who can take a look at your specific situation and advise you with the best option for you and your team!
Hope this helps!