Can a person teach a language without yet dominate it?

Initial clarification: I am talking about a very informal teaching style. Teaching in a formal environment without qualification would be irresponsible.

A bit of context. I offered free German lessons to two close friends of mine because of two reasons: to practice my teaching skills and to deepen my knowledge of the language. It is said, that when you can teach a subject, you have to previously understand it well enough. And when that is not completely the case, teaching is very helpful in solidifying knowledge. As an intermediate German learner, I find experimenting with this idea quite attractive. 

I just wanted to know your opinions. Will teaching with my current knowledge be effective? How much does one need to know about something to start teaching it at the basic level? I have this theory, that for example, to teach how to add, you don't need to know calculus: you just need to know how to add.

Let me know how you think it will turn out!

Personally, I'd rather learn the items you mentioned on my own, but what you describe is pretty common in language exchange so there is a precedence. 

I completely agree Marcos. I will be doing my best to teach in a structured way so they can make the most of it. My learning process will probably come from both preparing the lessons and realizing how much I know when teaching. I have known these friends for almost a year and they are amazing at giving me feedback and at being interested in stuff, so the sessions are probably going to be very fun and beneficial for all of us. If they turn out good enough, I will probably be posting my experience/findings through the process.

Also, as you said, I am not charging anything because it would be unfair. I am not an expert, they are aware of that as I clarified it, I also often tell them how my studies are going. It would probably be more accurate to call it a “study session” --or something along those lines-- in which I take a guiding role.