Here’s a little piece of javascript you can run in the console to assign all usergroups to a user in the V1 (old) interface. just press f12 to open the devtools, go to console, copy/paste the script, enter and don’t forget to click the save button to save the changes.
The script :
var aa= document.getElementsByTagName("input");
for (var i =0; i < aa.length; i++){
if (aa[i].type == 'checkbox')
aa[i].checked = true;
}