I suggest adding a filter/search field to the contact list plasmoid.
It should be placed at the top, have the same width as the list itself and always be shown. Do not put a label next to it, just put a hint "Search..." in it while it's empty. It should use instant (progressive) filtering/searching and have the "clear text" icon shown when text is entered. 
If the plasmoid supports groups and text is entered into the search field, auto-expand groups containing matching contacts and completely hide all other groups, so that nothing but matching contacts and their group headers are shown.