PhD in Germany after Bachelors
Essentially, you cannot start a PhD program without a master's degree in Germany. Some programs feature a dual-degree option, whereby you're admitted to a master's program as well as a PhD program. US and UK programs do basically the same thing, which is why the master's degree is not a prerequisite—you earn it, or the equivalent, along the way to the PhD after the bachelor's.

However, in German universities, most PhD positions are actually government jobs in what is known as the TV-L system. More precisely, it's what's called a "TV-L E13." Now, one of the requirements of the TV-L E13 is a master's degree in the appropriate subject. There really isn't a way to waive this requirement. 

What perhaps can be done is to ask if the group is willing to sponsor you as a "Wissentschaftliche Hilfskraft"—essentially, a part-time worker in the group—while you complete the master's program. (If they're eager enough to hire you with a bachelor's, then they should be willing to support you while you get the necessary training.)