Admin is a buyable rank, not a staff rank. It contains several perks, one of them is the /fly command in some servers.
Admins can only allow fly for themselves, not others. Otherwise, anyone could just pay an Admin in-game currency and/or items to get free fly and wouldn't donate for the Admin rank (which is not a staff rank by the way). However, it is a popular scam to watch out for. Many Admins will trick you into "buying" fly.
No one can give you fly. Admin donators receive /fly in Pvpfest. Anyone that says give me money and I'll give you fly is trying to scam you for money. Thread locked.