朋友

复数“朋友们”

For “a friend”, we say “朋友”“ 一位朋友”.
For “friends”, we say “朋友”or “朋友们” depend on contexts.
In most situations, it is save to use only “朋友” without “们”, because it is grammatically not correct to us“们” in same of the plural cases and it is optional in most plural cases.

The only case I can think of right now where you MUST use “们” is when you address to your audience before a speech. You MUST say “朋友们” or “各位朋友”(meaning every each of my friends).

Examples:

I have a lot of friends. 我有很多朋友。( Cannot use “们”here.)
Some of my friends are Chinese. 我的朋友中有些是中国人。(Cannot use “们”here.)
My friends say that I am a good man. 朋友们说我是个好人。(“们” is optional here.)
Hi friends, I have a good news. 朋友们，我有个好消息。(“们” is a MUST.)

For an image of these characters, check the following link.