I saw this stated somewhere and I didn't find it here, so I thought a formal feature request on this meta was necessary.
The Physics Overflow users page currently displays the usernames of all the users, whether Physics Overflow native users or Stack Exchange users, whose posts are in Physics Overflow. This is not a requirement of the SE attribution guidelines but it is nice to have.
However, the users list seems to imply, for a casual visitor, that all the users listed there are active users of this site and it qualifies, almost, as an endorsement. This is particularly important for SE users who post under their real names, or who post in Physics.SE under pseudonyms which they use widely. This is the case, for example, for Matt Reece, David Bar Moshe, Piotr Migdal, Pavel Safronov, G. 't Hooft, and Peter Shor, to take the first few cases in descending user ranking.
In my understanding, this is a professional site that hopes to provide a quality space for high-level discussion of physics. As such, it should reflect this professionalism by avoiding the misrepresentation that e.g. Peter Shor or Gerard 't Hooft are around this site and likely to post here.
I know that one can see whether each individual user has registered here or not by clicking through to the profile; for imported users who have not registered here, if you know where to look, the text "(This user is not yet participating on PhysicsOverflow, but we hope he will)" is present. However, it is too much to expect of a casual visitor to click through to all profiles to see how many of those users are actually available.
(I should note, at this point, the evident language bug. I hope this can be changed, as the gender imbalance in physics is already bad enough.)
The reason I feel that this is a unprofessional misrepresentation is that these users have done nothing to appear here except post in another site. While one hopes they will register and contribute here, they have yet to do so. Would it be OK to create an account for, say, François Englert, or Peter Higgs, and assign them a lot of points? That is evidently a misrepresentation as they have contributed no content to the site.
On the other hand, the Stack Exchange users on the users list have contributed content that is present here: they made their content publicly available on the CC-BY-SA license when they posted in SE. The users page does currently list people who have (directly or indirectly) contributed to this site. It does make sense, then, to have them present there.
There is an obvious middle ground here. Have non-native users present in the users page, but gray-out, or dim down, their entries in a way that makes this visible. For SE users who have not registered here, it is also a courtesy to link to their SE profile on their PO profile stub, and there is little reason not to.
In the end, of course, it is up to the moderators of this site whether to implement this feature or not. However, I hope that this will be implemented in the spirit of civilized and professional discussion that this site, as I understand it, aspires to instantiate.