I've successfully created a stick drawing of a tubular frame using 3D sketches. I'm now trying to use the structural member tool in the weldments section to convert the 3D line sketches into a truss.

I've created my pipe profile. I click the structural members tool. I then start selecting lines to convert. After selecting a few lines I'm unable to select more. Clicking the lines just gives no response. However, in the group box, if I create a new group, then I can select more lines to convert.

Question is, what are these "groups" when creating structural members and why can I only select a few lines in each group?