You have two options to expand the storage space for a team.
You can earn up to 10 GB of storage space for free. In this case the storage space is always associated with the team you own, which means you are the Creator of the team. To learn more about how to earn up to 10 GB for free, please read the following FAQ.
The second option is to purchase Additional Storage plan and get 100 GB for $9/month. This purchase can be done by all Administrators in a certain team (including the Creator who is an Administrator by default).