Pixelic's simple pricing is based on the number of active members on Pixelic. For every active member on Pixelic, it's $12 per month.
Active member means the user has an active account with Pixelic and is a member of your workspace.
(Note: to know who's on your workspace and manage memberships, you can go to Manage Members on the top right corner dropdown menu.)
Anyone who's NOT on Pixelic as a member (eg., your teammates on Slack or Figma) does not count as an active member. Since they do not have an active membership with Pixelic, you do not have to pay for them.