How can I grab a loop or group of vertexs and assign them a weighted value?
For example: The shoulder of an arm needs to be 50% at the joint, but between the shoulder and the elbow is entirely 100%. Same goes between the elbow and wrist.
If I could auto-fill the values between the shoulder/elbow and elbow/wrist would save me tremendous amount of trouble.
You can do it with Vertex groups (which is all weight painting is)
Go into edit mode, and select the required loops, and then in the Object Data panel, find the vertex group for the bone, or whatever, for that particular piece, adjust the value/strength, and hit assign.
Should be that simple 
Magnavis is right, but it sounds like you want to fill in multiple loops, with automatic interpolation. I agree, this would be a nice way to do things, but it isn’t quite possible with Blender.
There are a few things you can do after assignment of the 1.0, 0.0 loops to fill in the middle. One technique is to use a weights->smooth operation (on the loops you want to fill only.) Another technique is to make a low poly out of those two loops (bridged, separated to a different object) and then data transfer the weights over with nearest face interpolated mapping.
Another thing to keep in mind is, it’s usually a better idea to weight something lower poly than to weight something higher poly. Even if you have more detail than you can show, a mesh or surface deform from a weighted low poly ends up being easier to do (and probably, better performing.) High poly weighting is challenging. Weighting is tons easier when there’s only like one or two loops between your 1.0 and your 0.0 weights, at which point you could just assign it.
If they are the same you can preweight one vertex manually and copy that to the others.