How to politely invite the abroad (some? potential?) friends from abroad, who want to exchange the language, to join in the group politely (I moved "politely" next to "invite", so that its clear that you want to "politely invite" rather than you want them to "join politey". You can still use your original word order, and its still correct. But because the sentence is long, its not that clear anymore which verb the "politely" describes.)

I have some friends learning Chinese in italki .Some of my university friends are also learning English. (I marked your punctuation in yellow, you made a little "typo") I hope we can help each other for improving improve the language (maybe better to say "our language skills" instead of "the language"). As Skype not doesn't work very well in China., I've build set up (or opened) a QQ group. But maybe some people will may/might think it is in bad manners for to send an QQ invitation for to join in the team directly. I don't know how to politely invite the abroad friends friends from abroad who want to exchange the language to join in the group politely. How What do you think? And do you want to join in? ("Join in" is ok, but in this context "join" is better and enough)