I recall seeing on Facebook group adding a user, then deleting when needed as a workaround for a temp support agent. We pay annually, and I just did this. Billing department is now telling me I now have a prepaid user slot for a year even though I currently don’t need it.
There is no way to have some users annually, and other users monthly. I am not sure any system allows for that, really. Your account is billed as one unit. So if you are annually additional users would be annual as well. When your annual expires, you could go back to monthly and then spin up and down users as needed on a month-to-month basis. It wouldn’t work if your account remains annual, though.