Raids are available to players in Guilds, and require a certain amount of Guild tickets to activate.
 
If your Guild has started a Raid and you missed the sign-up period, or if you don’t have any units at the required star level to participate, you won’t be able to take part in it.
 
We no longer enforce a waiting period on players who have recently switched guilds, so you shouldn’t see any problems joining because of that.