Give op/staff right's to a person you don't trust is not smart!
For all you know they could be griefers. They would come like ants.
TIP: Only give it to people you really trust.
And if it's to get friends it's also a bad idea :)

Invite people to your server istead and post some info about the server.
Watch how they behave.
Let them play there for a while.
Do they help others.
Can they be trust worthy?

Some tips that would make it better.