With HubSpot Sales and Service Hubs, any tier above free is on a pay-per-seat model. This means that in order to access certain tools and functionalities in your portal, users will need a paid seat.
Each time you give a team member access to your HubSpot portal, you are assigning them a seat (one user = one seat). If users in accounts with paid Sales Hub or Service Hub subscriptions have not been assigned a paid seat, they will not have access to paid subscription features. In simple terms, for team members to use paid subscription features they have to have a paid seat.
Note that paid seats are not relevant for Marketing Hub