Is there an example of using group.memberParts?

What I need is to get all node in a group, will memberParts give me the nodes?
can’t find an example of how to use it?

Yes, iterate over the Group.memberParts collection. You may want to skip over any Parts that are not Nodes, depending on your purpose.

  aGroup.memberParts.each(function(p) {
    if (p instanceof go.Node) { . . . }

Read GoJS Collections -- Northwoods Software for more examples and discussion, or search the samples for memberParts.