add(index, weight, type)
Add vertices to the group

Parameters:
•index (int array of 1 items in [-inf, inf]) – Index List
•weight (float in [0, 1]) – Vertex weight
•type (enum in ['REPLACE', 'ADD', 'SUBTRACT']) –
Vertex assign mode
◦REPLACE Replace, Replace.
◦ADD Add, Add.
◦SUBTRACT Subtract, Subtract.