The person may not have full knowledge or full consent in this "permission" --- i.e. a kid could be drunk and say "OK, I'm on Satan's team" or just half-jokingly perform some stupid act of sacrilege --- but basically, Satan cannot overpower and possess a person completely against their will.
Satan can "oppress" or "attack" an innocent unwilling person, but he does not have the power to "possess" an innocent unwilling person.
Most evil has got to be the result of the ‘”oppress” or “attack”’ kind of diabolical influence; true, all-in type possession may be fairly rare, who knows. I’m pretty sure exorcism puts the kibosh on all of these. Probably every single person other than Mary has at some point done something or entertained some thought that the devil could construe as an invitation for at least some dirty dancing, if not full possession.