Hello and thank you for your question.  Aspen tree groves are easy to let get out of hand if you aren't paying attention.  I'm happy to discuss this issue with you to find out what you are trying to accomplish with this "managed" growth.  Please give my office a call<personal data hidden>.

I am trying to cultivate a new grove. The clusters are between knee and thigh high. Should I prune the weaker shoots, or leave them alone until Spring?
Also, Any thoughts as to what is going on with the  bark of one of my mature Aspens?

Aspen are sensitive to many environmental factors and will exude sap when it is stressed.  I wouldn't be too concerned unless you see other indicators of stress such as premature leaf drop, or droopy leaves.  You can cut the bushier saplings, a strong aspen will have a single stem and gain height within the first few years.  If you are trying to encourage your grove, I would look for those trees and cut out the rest.