point

int  setpointgroup(int geohandle, string name, int point_num, int value, string mode="set")

primitive

int  setprimgroup(int geohandle, string name, int prim_num, int value, string mode="set")

vertex

int  setvertexgroup(int geohandle, string name, int prim_num, int vertex_num, int value, string mode="set")

引用・参考

https://www.sidefx.com/docs/houdini/vex/functions/setpointgroup.html