Staff
List the people who run your activities, then switch ON, for each of them, the activities they can run. Accommodation is not concerned: only activities that need people (set in the Resources tab) appear in the table.
The plugin only counts people, it does not name one on each booking. With one person able to run two activities, those activities are never offered at overlapping times; with two people, they can run at the same time.
Absences
Add a period of absence (holidays, leave) to a person: they are not counted on those days, so fewer times are offered. Bookings already made are never cancelled by an absence. A person set to inactive is not counted either.
Warnings
A notice appears when an activity needs people but nobody is ON for it, or has no duration: in both cases it cannot be booked online until you fix it.