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It sounds like these people are squatters. You say the house belongs to your grandfather, so your grandfather certainly may object to these people's presence. If you have authority to act on behalf of your grandfather, you may give notice to move out; if they fail to move out, you can bring an unlawful detainer action to evict them. Be sure to consult your own attorney to protect your legal rights.
Consult a local attorney. I would simply change the locks. They do not seem to have any legal right to stay there. There was no lease or contract at all.
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