The Raids have begun. The multiplayer feature that was included in the last update has gone live. Raids will now appear alongside your nearby Pokemon scanner. When a Raid is due to begin it will appear in the Raid list with a time. The time is long long before the Raid is due to begin, giving you and your friends time to get their and prepare.
In order to participate in a Raid trainers will require a Raid pass. These are given out 1 per day by spinning the disc above a gym. Extra ones can be bought in the shop. Completing Raids gives you bonus rewards including golden Razzberries and Rare Candies as well as a chance to capture a higher than normal rated Pokemon.
Raids currently come in 4 tiers of increasing difficulty (1-4) with 1 being the easiest. The rumor is there is a 5th Tier but this will be reserved for Legendary Bosses. This is as yet unconfirmed. The higher the Raid tier the more people will need to participate to have a realistic chance of defeating it. Lower tier bosses will require less people and may be able to be taken down solo.
The great community at TheSilphRoad have done some analysis and found what Pokemon Bosses appear in which tiers and they have also got a good idea of the Boss’ CP you will be facing. Here are the 4 Tiers and the Pokemon Bosses in each.
Pokemon Raid Bosses