public void addToGroup(final T element, final int group) {
        int index = groupStart(group);
        if (index < 0) {
            final Map.Entry<Integer, Integer> higherEntry = this.groupToStart.higherEntry(group);
            index = higherEntry == null ? size() : higherEntry.getValue();
        }
        addInternal(element, group, index);
    }
Adds a new element at the beginning of the group area for given group.
If the given group doesn't exist yet, it is created in the right place and element is added there
@param element element to add
@param group group to assign onto element
main/src/main/java/cgeo/geocaching/utils/GroupedList.java