Nintendo recently added the Raid Battles feature in Pokemon GO wherein you can possibly defeat a Raid Boss or capture a higher ranked Pokemon. However, players will need a raid pass to participate in the event and these passes cost you a fortune.
You can buy a premium pass for a one-time raid, but it will cost you a lot. So, if you are looking for a free option then there is a nifty little trick.
Obviously, you are going to spin a Photo Disc at a gym to acquire a free raid pass. But, one raid pass allows you a single entry into the Raid Battles in Pokemon GO.
The catch here is that you need to use up your unused or saved raid passes, before you spin a Photo Disc. Otherwise, you will not be eligible to get the free raid pass for the day.
The game does not offer an option to carry over unused raid passes. So, the best option is to use them up and then get another free raid pass.
In this way, you can use at least two raid passes per day to join two Raid Battles. Remember that you cannot hoard more than one raid pass for several days in a row and then use them all together.
As the Raid Battle system for Pokemon GO is still in its beta stage, we could only hope that the final or public release will not have these limitations. Nevertheless, you need to be level 25 or higher to participate in a raid.