In Groningen you should board the interliner (=light blue bus-service) to Emmen. It stops in Borger. It takes about 3 quarters of an hour.
It is a bit more expensive than the regular buslines but it's a lot faster/more comfortable and you don't have to change in the middle of nowhere.
Groningen is worth a visit too. It's the only real city in the Northern part of Holland.

Have a nice holliday.