From their order I'm inferring you're using get_defined_functions for this.
One way to group them might be by extension. For each function in turn, create a [man]ReflectionFunction[/man] object and ask it for the Extension name.
You could then sort the list alphabetically by function name, and use the extension name as a class. Then the user can use a dropdown to select an extension, and have jQuery show all of the functions that are part of that extension while hiding all of the others. Bonus points for styling the dropdown so that it doubles as the heading for the list (instead of having to have the list of extensions twice - once in the dropdown and once as the headings).