You have been surounded by very good friends.

被动当然用by 了 by 后面是发出动作的人啊。
have been 是现在完成时态。翻译成：你已经被非常要好的朋友包围了。

此句为被动语态，by后跟施动者，surrounded by very good friends被一帮好友围绕（直译）。have been,现在完成时，表示过去发生的某件事一直延续到现在。

You have been surounded by very good friends.是被动语态，用by表示施动者。
have been 现在完成式。