More details from IGN websites
http://www.ign.com/articles/2018/06...-get-trading-soon-alongside-new-friend-system
Raising your friendship level will offer various bonuses, including reduced Stardust costs for trades, extra damage during gym battles alongside a friend, and additional Premiere Balls during raids alongside a friend.
These are the bonuses for each friendship level:
•Good friend: Trade all Pokemon except Mythical
•Great friend: Stardust discount for trading, attack bonus during gym battles with that friend, one additional Premiere Ball for raids alongside that friend
•Ultra friend: Additional stardust discount for trading, larger attack bonus during gym battles with that friend, two additional Premiere Balls for raids alongside that friend
•Best friend: Major stardust discount for trading, largest attack bonus during gym battles with that friend, four additional Premiere Balls for raids alongside that friend
You can increase your friendship level with a particular friend by sending gifts (see below), trading, or participating in gym battles or raid battles together. Your friendship level can only increase once per day per friend, and Koa explained the following ranges for achieving a friendship level:
•Good friend: 1 day
•Great friend: 7 days
•Ultra friend: 30 days
•Best friend: 90 days
The current cap for each player’s friends list is 200 friends, but Koa said that number could grow in the future, and Niantic will never require players to pay for additional friend space like with Bag or Pokemon storage.
You’ll also be able to participate in a Special Trade, which is for Legendary Pokemon, Shiny Pokemon, or Pokemon not currently in your Pokedex. You can only make a single Special Trade per day (not one per day per friend – only one per day total). Koa clarified that this “once a day” limit does not refer to a 24-hour period, and confirmed that it will reset at midnight like with Catch and Spin streaks.
All trades will require Stardust, with massively reduced costs depending on your friendship level. An example in screenshots released by Niantic shows a 1,000,000 Stardust fee for a Special Trade, reduced to just 40,000 Stardust if the players are Best Friends. Koa confirmed that both players will pay the same Stardust fee for each trade.
Players will also receive candy bonuses depending on the original catch location of the Pokemon being traded. If the Pokemon were caught far apart, you’ll receive extra candy. The current upper limit on these bonuses is a 100KM distance between the Pokemon’s original locations.
Finally, the HP and CP of Pokemon traded to friends will be reset, resulting in random new IVs for the Pokemon you receive. The range of possible HP and CP outcomes can be extremely wide or extremely narrow depending on your friendship levels,
with Best Friends much more likely to receive stronger Pokemon.
PokeStops will now occasionally drop Gifts, which you can send to friends on your list. Gifts will include a “postcard” from the PokeStop where you picked up the gift, as well as a rare item. Koa clarified that these items include rarer items within the normal PokeStop pool (hyper potions, revives, etc.) and not raid and research-exclusive items like Rare Candy or TMs.
When a player receives a gift, they’ll also have a chance to receive a new 7KM egg, which will eventually hatch to reveal an Alolan Pokemon.
The particular Pokemon in these 7KM eggs will be exclusive to hatching, though this will not be the only way to receive Alolan Pokemon moving forward -- other Alola forms like the previously-released Exeggutor and the recently-announced Diglett and Geodude will still spawn in the wild.