If employer reopens at reduced capacity can they selectively NOT rehire high risk group workers

No.  I do not believe the employer could unilaterally decide that someone should not be re-hired because they are in a high risk category.  This is especially true given that those in a high risk category are typically disabled or in a protected age group (40+).  In other words, these people are all in protected categories and by refusing to hire them, you would be discriminating against these individuals.    While the ADA permits employers to exclude employees with a medical condition that would pose a direct threat to health or safety, I don’t see how these individuals could be deemed to meet this threshold.  These individuals are simply more likely to suffer catastrophic effects from the virus.   They, themselves, are not a threat to the health and safety of others.

If you, as an employer, wanted to offer these individuals a reasonable accommodation in order to reduce the likelihood of harm, that’s certainly allowed.  It will be a rare circumstance where there is no reasonable accommodation available given that telework or leave are generally accepted reasonable accommodations.