Technically anyone, but it might constitute treason for citizens of at least one country.

And they should get all the more respect for it if they do anyway.

We've said they're Ukrainian in several places. We avoided being explicit about it for the initial announcement to focus on getting help. We avoided specifying it to avoid our post being perceived as a political statement against Ukraine or attracting a bunch of negative attention. We only got targeted with an insignificant amount of additional attacks due to this announcement because of how it was worded. It's fine to simply say it's Ukraine now instead of only heavily implying it.