This functionality has been added to master, 
The Assignments Tab will be viewed on the group (/g/:groupname) page and will be visible to staff, and anyone in the assign allowed groups setting.
Here are some screenshots:
Everyone tab shows assignments of all group members
username tab shows assignments of a single group member
Thankyou @david for being such a great mentor, helping and supporting me doing this task 
Future work:
- We’re planning to add per-user assignment counts to the sidebar and sort the list in order of assignment count.