Only the owner can change the number of seats for the subscription in the Billing panel.
Increasing and decreasing number of seats
You can adjust the total number of seats on your account to increase or decrease the amount of active users by selecting "Add Seats". When you add or remove a user, the subscription doesn't automatically update to reflect the current number of active users so you'll need to update this in your billing panel to reflect these changes.
Adjusting active seats
If the maximum amount of active seats has been reached, you can purchase additional seats in the billing panel or remove another team member's seat to free up a seat. Removing a user's seat does not remove them from the team, only their access to use Interseller's functions such as the Chrome extension and the email sequencing function. After removing a seat, you can then invite another user to join the team.