My grandfather recently passed away, is there away for me to add myself to his tree?

Hi there, My grandfather recently passed away. We know very little about his side of the family. However, he has created this huge family tree that gives us more information than ever. I am wanting to add my grandfathers children so on and so on to the tree that he has created. Is there away for me to get added to his tree? No one else has his log in information. Is there away to do this? Thank you.