below-grade concrete basement , 5/8 bamboo. What do I do float or
glue down.

What does the manufacturer say? Not all floors can be successfully floated. However, IF the manufacturer says that the flooring you have can be floated, then that would be your safest method as you can install an 8 mil polyethylene vapor retarder and lap it up the walls a bit. If you have to glue it down direct, make sure you use a "moisture blocker/barrier" system warrantied to reduce moisture vapor emissions to acceptable levels.

I'd be leary of both, being it is going below grade. Your best bet is a floating install over a moisture barrier and leaving plenty of expansion gap. More then they require. As that will be a humid environment, at times.