Back to the original question. There is no such thing as a Group A, it was replaced by a Single Engine Piston SEP rating in 1999, and is now in your licence not your log book.
To fly a motor glider you have two options obtain either a TMG or a SLMG rating much as described above, involving training plus a test.
I recommend the TMG rating because it is valid for 24 months. Once you have this and your SEP rating, they can both be revalidated using the same flights on either Class of aircraft.
The SLMG rating is valid for 13 months and has to be revalidated seperately.