Pokemon-go – From what range can you see a Raid Battle

There's no way to battle someone else other than the Asynchronous Gym Battles. But this may change in the future versions of the game, as it's a very expected feature in any Pokémon game.

To receive the defender awards, you must have at least one Pokémon in an allied gym.

To collect the rewards for having a Pokémon at a gym, you have to go to the shop while at least one Pokémon of yours is still defending at an allied gym. Thus it is best to capture as many gyms as possible (up to 10) and then collect before any get taken down. To get to the shop, from the home screen tap the Pokéball icon, then tap "Shop". Then inside the shop, there is a button in the top right where you can collect rewards for having Pokémon in a gym. Note that you can only collect once per 21 hours.

Also, as pointed out by Virusbomb, "the rewards for defending a gym are 10 Pokecoins (which can also be bought with real money and are used to buy in game items) and 500 stardust. You can get a max of 100 coins and 5000 stardust every 21 hours with Pokemon in 10 different gyms."