Every now and then it's a good idea to review your users and check who isn't using the site; this will free up some user licenses, enabling you to invite more people into your Kahootz site! Site owners can look at who isn't using the site, via Account > Site Admin > Users. If you sort by the Last Active column, with the arrow pointing down, you can sort your members into least active - to most active. The activity date is set when people access the site (it's not specific to any particular workspace) or if they send an email. Last Activity is also seen on the user's profile, found on Site Admin > Users > then clicking on the user.
Please note that it is possible for people to be usefully receiving notifications from the site but not needing to do anything online - you may want to think if such "passive" use is relevant for your site.
You can also check how many workspaces people are members of, or invited to, using the Workspace Count column. Again, please note that if you have some open or restricted workspaces on your site they may be using them as a registered user but not marked as a member.
To review Invited users who haven't yet taken up their invite, click on the Show: Invited Users control above the list. You'll be able to list them by the date they were last sent an invite or reminder. You may be able to spot outdated invites (workspace count will be zero), or mistyped or bouncing email addresses on this list.
Once you have an idea of when your members last used the site and how many workspaces they are a part of, you can decide what the action will be. You may choose to remove people who haven't been active for long periods of time, or members that are not a part of any workspaces.