In the current time of globalization, collaboration among people in virtual environments is becoming an important precondition of success. This trend is reflected also in the educational domain where students collaborate in various short-term groups created repetitively but changing in each round (e.g. in MOOCs). Students in these kind of dynamic groups quite often encounter various difficulties, which are obvious mainly when students' characteristics do not complement each other. In spite of various group formation methods aimed to solve the group compatibility problem, most of the existing approaches do not consider dynamic groups. We describe (i) a proposal of a novel group formation method based on Group Technology approach, which considers feedback on students' collaboration to improve group formations; (ii) an application of the method as a part of a collaborative platform PopCorm, which provides students in the created groups with a set of real-time collaboration tools; (iii) a long-term experiment in which the groups created by our method achieved significantly better results in comparison with the reference approaches. Our results indicate that considering feedback from students' collaboration can improve the group formation process as the groups created by our method achieved higher collaboration quality with next iterations.