Set up a whitelist and operators

Type these in the Console (without a leading slash).

Whitelist

whitelist on
whitelist add PlayerName
whitelist remove PlayerName
whitelist list

Also set enforce-whitelist=true in server.properties so players who are already online are removed when they lose access.

Operators

op PlayerName
deop PlayerName

Operators have wide power. Prefer a permissions plugin such as LuckPerms and give staff only the permissions they need.

Bans

ban PlayerName reason here
pardon PlayerName
ban-ip 1.2.3.4

Letting staff manage the server itself

If a friend needs the console or files, don't share your password — add them as a subuser with limited permissions. See Giving friends access with subusers.