You can easily manage the number of "seats" associated with your account by logging in and navigating to the icon at the top right of your screen and clicking to select "my account". You will see a tab for "my subscription"- you are able to add or manage seat count there. Please note, you can adjust users (swap one user for another, for example) without adjusting the overall seat count until renewal time! At that point, any active seats will be part of the renewal of your account.
Once seats are added, navigate to "my team" at the account icon top right of page. There you will be able to add or update users!