Not a teacher.
The simple past continuous also happend in the past and is ended. You stress with the progressive form the duration of your work and not only the fact that you have worked. So, when the meeting was on Friday, you were busy working and couldn't join.
"Had been working" wouldn't fit here because your work and the meeting on Friday happend at the same time in the past and therefore you weren't able to join your friends.
Please correct me if I am wrong. Corrections are always appreciated.
Interested in Language