Adding members to a specific group, from a NAB view

You mean, instead of opening the Group document, click the down-triangle after the Members field and select the members you want?

It can be developed, but currently there is (afaik) no such button available.

Any idea about a script i could use ?

I've done this from the uiworkspace on a view:

      Dim session As New NotesSession
      Dim db As NotesDatabase
      Dim workspace As New NotesUIWorkspace
      Dim uiview As NotesUIView
      Dim col As NotesDocumentCollection
      Dim doc As NotesDocument
      Dim msgPrompt As Variant
      
      Set db = session.CurrentDatabase
      Set uiview = workspace.CurrentView
      Set col = db.UnprocessedDocuments
      Set doc = col.GetFirstDocument
      
      msgPrompt = workspace.Prompt( PROMPT_OKCANCELEDIT, "Type Chapter/Section", _
      "Enter the Chapter/Section Title for the selected documents." )
      
      If Not Isempty( msgPrompt ) Then
            While Not(doc Is Nothing)
                  doc.Chapter = Cstr(msgPrompt)
                  Call doc.Save( True, False )
                  Set doc = col.GetNextDocument(doc)
            Wend
            
            Call uiview.DeselectAll
            Call workspace.ViewRefresh
      End If

With the above code, you select documents in the view, click the action button and then type in the value to populate "Chapter" field on the documents.  You could even do a lookup to the groups view so the list of values in the prompt is a list of your existing groups.

Let me know if you need more help.