You obviously need a "Remote raid pass", but by free, I mean you don't have to pay for additional charges which are requested by other other similar apps. But if you download the app "Raid plaza", you can get invited to participate in a Necrozma raid.
All you have to do is download the app and watch a commercial. You have to watch the commercial to get 1 coin, and you need 1 coin to register yourself in a pool of an available raid of your choice.
If you registered for a raid, switch back to the Pokemon Go app. I had of course registered for the Necrozma raid, and I had to wait less than half a minute in total before I received a friend request.
I accepted and was immediately invited to the raid.
I was skeptical at first, because there were only 3 other people, and 2 had usernames that were very similar, but eventually the raid was cleared with 10 people.
It was all quite simple and recommended if you don't want to pay the extra costs of other apps.
by Male_Parent