I am not sure whether I fully understood the question but it seems like you have assigned same beams or elements to multiple groups and would like to get a list of all the groups to which the particular beam/element has been assigned to. If that is your question, the answer is there is no direct way to do that. However there are couple of things you can try out. You can go to the editor ( Edit > Edit Input Command File ) . Go to the section of the command file where you have the group definitions and the use the find/search tool from within the editor to search for the beam/plate number. The search would find the beam number. Press F3 to find subsequent occurrences of the same search term. That way you would be able to find all the group names in which that beam/element appear. An alternative graphical way would be to go to Select > By Group Name, select the groups and visually check what are the groups for which the particular beam/element gets highlighted.
If this does not answer your question, please explain in a little more detail as to what you are trying to do with the groups and I will try to answer.
If this does not answer your question, please explain in a little more detail as to what you are trying to do with the groups and I will try to answer.