When you purchase a paid plan for your workspace, you’ll be billed for all active members. Following Slack's Fair Billing policy, you won't be billed for any members that are inactive or deactivated. 

Inactive members 

For billing purposes, an inactive member is someone who hasn't used Slack in over 28 days. Slack will automatically detect if members become inactive. 

Inactive members still have access to your workspace and paid features, but you'll only be billed for them if they become active again.
 

Deactivated members 

A deactivated member no longer has access to a workspace. Someone can deactivate their own account if they’d like to leave your workspace, or you can deactivate them.

If you do not want to be billed for a member, we recommend deactivating their account. Deactivated accounts must be reactivated to be billable again.
 

See a list of members by billing status

A Workspace Owner can see a list of active, inactive, and deactivated members: