No, and deliberately not, in both directions.
- Adding a seat costs nothing today. The new amount starts at your next renewal.
- Removing a seat does not refund. That person keeps access to the end of the period you already paid for.
Why not
Because proration would quietly break the one promise built to prevent surprises.
When you approve a seat, the screen tells you the new monthly total and the date it starts. If a partial charge then landed on some other day, that sentence would be false, in the exact mechanism designed to stop unexpected charges. One date, said once, on every screen, is worth more than a few dollars of precision.
Who comes out ahead
Mostly you. Add someone the day after a renewal and they are in for most of a month before the charge changes.
It runs the other way on removals: take someone off and you have still paid to the end of the period, so they keep access until then. Removed is not the same day as gone, and if you change your mind before that date you can switch them back on with nothing having happened.
Upgrading and downgrading
The same shape. Changes take effect on the renewal date, and the screen shows you that date before you confirm.
If a charge ever arrives that you cannot line up against a date you were shown, tell us. That would be a bug, not a policy.