I want to get some new experience working with different people around the world. So, I'm looking for some sort of a collaboration site, where people looking for designers/developers etc and collaborate in order to create it. This article is dedicated to game development, but what I'm looking is some iOS\android developement with backend. Any help?
Why trying to find a team? If you have an idea, try to gather a team.
Use forums, present your "practice" idea and gather a team of international developers who will work for free. There are many guys ready to do this to gain some practice.
You can even try posting a project on freelancing service websites asking to gather a team for collaboration.
The bottom line is that you are not bound to join others' team. At this very moment there are millions of developers wanting to gain some experience.
Have you tried looking at open source projects on GitHub? Or create your own if you think you have an idea. Open source projects are usually open for collaboration. Find one (maybe a small one to get you started) that interests you and ask if you can be a (outside) collaborator. It will also give you valuable work experience working with Git and GitHub, on top of code structuring and style and the subject at hand. And it's all in the open, so it makes a great addition to your resume.
Otherwise post a call on LinkedIn, perfect platform to find other like-minded professionals.
Ps. I'm in the mobile game development business and "we" need back-ends too... Lol.
You can also try community hubs. There are many Meetups, facebook groups and forums where developers gather around some specific technology they use.
You could try interacting in a few of them and try to find people to work together.
Going straight for Github as @stevenvanc suggested is a good idea too.