Slack only bills you for people who actually use it.
You get credit back when someone goes inactive.
The What
On Slack's paid plans, you pay only for active members, and Slack spots inactive ones on its own. If someone you paid for stops using Slack, you get credit back, prorated, for the time they didn't use.
Paid Slack plans let you add a contractor, intern or client to a single channel as a Single-Channel Guest, with no seat fee. Each paid active member lets you add up to 5.