This is actually easier than you think. However, since I found the solution only after a long search on the Internet, I write it here briefly:
One sends an e-mail to: [group name]+subscribe@googlegroups.com. This can be empty.
After that you will receive an e-mail in which you have to confirm your membership. You can do this by replying to the e-mail or by sending an e-mail to [group name]+subconfirm@googlegroups.com.