Working on Solidworks 2011 if I open a beam study and right click the "Joint Group" and go to "edit..." then push the "Calculate" button. Select "display neutral axis" (that is optional but it helps to know what is going on). Finally you should be able to right click an existing node and a selection box will pop up that will allow you to select or deselect the members that are part of that node.
The documentation talks about being able to click on things in that box and hit the "delete" key to remove them, this doesn't work. If a box is up and a member is pink then it is "connected" to the node, if you want to remove/add it simply click the model on that member and it will be removed/added.