Importing Contacts into multiple groups
An alternative approach, esp if you have comma separated options for the Groups (ie all groups are in a single cell) would be to make a Custom Field with options that match your Group Names, import like that, then search for each option and via Actions 'add to group'.

Or (as Pradeep suggests in comment below) Custom field of type text and then using Search builder perform Like %Group Name% for each option and via Actions 'add to group'