How to add file to a group?

OK. In case you don't find the button... (I assume what you actually mean, is that the feature is for some reason currently not working on Linux, but I can't verify that right now)...

There is two ways to add a file to a group. Either right click the file itself. Or click on the function in the bottom pane of the window, when the file is selected. The file has to be public, if you want to add a file, which does NOT belong to you (that is a file of a friend of yours). Your own files can be added also to private groups. Wuala will ask you to change the sharing settings accordingly.