I wanna split my correction in three parts (as you can see below).

Part A  Punctuation

First of all, I wanna give you some tips about the Chinese sentence constructure, to be more exact, the punctuation. Actually, it is pretty much the same as that of in English. When you have done talking about one thing, or the sentence next to each other are highly related, it is better to use a comma rather than a full stop. Use a full stop only when you are have done talking about one thing. The exact examples are in the correction below.

我刚刚参加了几个交流团

我刚刚参加了几个交流团。,我觉得那些团很有用的。

（In the above sentences, sentences are related to each other by the object "团", so you should use a comma in between, instead of a full stop.)

为什么呢？因为它们让你找到会说你要说的朋友。,他们都可以帮你学。

（The same reason, sentences above are connected to each other by the object "朋友". You use "他们" in the second sentence to refer to "the friends that can offer you the help you need", right? In this case, you should use a comma in between, instead of a full stop.)

在我的大学，上课时，我们分组的时候是我们班最有用的。

(The punctuation is this sentence is correct.)

特别是说话课。,每一个说话团都有不同的题目。

(Here, the two sentences are related because of the object "说话团", so put a comma in between.)

我觉得如果你觉得你学得有意思呢，你肯定学得好。

(The punctuation is this sentence is correct.)

Part B  To write more native.

I wanna show you the sample that I write so you can see what phrase I would use and how I would organize the sentence if I were you.

Part C  Tips

Sometimes, the different expressions only differ in minor aspects, a word, a pharse, or even only a puctuation, but the meaning or the feeling in the sentences you make can be a great difference. Use your heart to experience the language, it will help you learn more quickly. XD

Feel free to ask if you have more questions.