Yesterday evening, my friends and I held (better -- "had") a party.

Because we didn't see each other in a long time. <-- incomplete sentence; it's a dependent clause. Thus, it needs another clause to complete the sentence such that: Yesterday evening, my friends and I had a party because we haven't seen each other for a long time.

All of us (or "we") were friends when we were (this is not necessary) in the middle school.

In the first activity, we first played the pool ball and then we played mahjong.

In my opinion, we get together so that we will be happy. (it's a correct sentence, but for the sake of the flow in your writing, I would rewrite it this way: I thought we got together to have fun.)

But I am was completely wrong.

As we all know, mahjong is a game which is played by 4 (usually, it's better to spell out numbers in between 1-10) people.

At that time, (prepositional phrase at the beginning of the sentence needs a comma) we had six people there.

So two of us couldn't do anything except sit around while the others were playing; furthermore, all of us don't didn't know how to play mahjong very well.

So after an hour later, the two who were sitting beside and the other two who were playing we thought this was boring and decided to play something else.

So one of us suggested to go on the net bar to play the computer game which was more interesting.

So he said he didn't want to go to/on (in English we use the preposition "on" when we go online to a webpage; however in your case, since this is a "net bar," I think "to" is okay as well.) the net bar.

If we went to the net bar, he will go home. ? You mean, "If we went to the net bar, he would have gone home." ?? So you guys didn't go on the net bar to keep your friend from going home? Or did he leave for real?

You know, all of us are studying in different places and it's hard for us to get together.