Supercell (the Clash of Clans developer) is currently looking into adding support for Google+ friends through Google Play Games, but this feature is not yet available. [1] X Research source
Open Clash of Clans and tap the Trophy button. Tap the Friends tab and then tap “Connect to Facebook. " Confirm that you want to connect accounts in the Facebook app or website that opens. You will need to log into Facebook if you aren’t already.
Open the GameCenter app on your iOS device. Tap the “Friends” tab at the bottom of the screen. Tap the “+” button in the upper-right corner. Search for your friends using their GameCenter nickname or Apple ID email address.
Tap the Trophy button in Clash of Clans and then tap the “Friends” tab. Tap the friend you want to invite. You’ll only see people listed who have also connected Clash of Clans to either Facebook or GameCenter. Tap “Invite” to send a clan invite. This option will only appear if that person is not already in a clan.
Tap the “i” button at the top of the screen. Tap the “Join Clan” tab. Type in the clan tag with a “#” before it. For example: “#P8URPQLV. "
There is no way to specifically choose which clan you are going to attack.
For example, Clan A may have four Level 10 Town Halls and three Level 9 Town Halls. Clan B may have four Level 10 Town Halls and five Level 9 Town Halls. You’ll have the most success if both clans have the same number of Town Halls at the same levels. Both clans should at least have the same number of the highest-level Town Halls.