I may have solved it; I need a student to test it to be sure.
If this helps anyone else...here is what happened.
Groups were set. In course setup, separate groups was selected and they were not forced. The specific activity - a forum - was set to separate groups. Students could see it, they could not reply.
After a lot of poking around and setting up new test forum activities, I decided to change the type of forum. That appears to have been the issue. Moodle allowed me to choose a Single Simple Discussion when I set it up and didn't warn me that this was incompatible. I didn't read the descriptions because I have regularly used this type of forum. When I decided to change that parameter, I saw a notice that this option does not work with groups. I changed it and when I change my view to that of a student, I can see a reply button and I couldn't before.
Also be aware that if you create the initial post before you set up separate groups, students can see the post but get no reply button because the original post belongs to no group though it is visible to all.
