Group Parts Tool

Hey guys, I am pretty new to SR2 and space simulators in general.
I haven't figured out the Group Parts tool yet, which would really be helpful for speeding up the detailed building process, so maybe you could help me.
Whenever I use it or press G as the shortcut, I get the popup info, that it couldn't transform one single part into a goup, but I can't figure out how to select multiple parts at once.
Thanks in advance :)