This is equivalent, and might be a bit clearer and more performant:
```python
return list(itertools.chain.from_iterable(group.user_set.all() for group in groups))
```
Alternatively, you can be a little less clever but a little more readable, and just do:
```python
ret = []
for group in groups:
    ret.extend(group.user_set.all())
return ret
```