E-Mailing a Professor about interest in his research group during application for a master and before admission(in Germany) I hold a Bachelor in physics and I am applying for a master program in Germany. There is a research group by a professor, with topics very close to what I'd like to study and furthermore work on. On the group's site it is posted that there are open master-phd positions.

Should I email the professor, expressing my willingness to study and possibly work in the future under his group if I am not yet admitted to the program?

My email would contain some basic information about my choices of classes, bachelor thesis, personal studies after my bachelor and interests in the research group.
It is usually the case that the information you intend to include in your email about your studies would be asked for in the application, so preemptively providing that would be unnecessary.

It may be useful to email mentioning your interest in the research group, and ask any questions about the research you may have.