How do you accept an invitation to the penthouse (GTA 5)?

I play online with my cousin GTA 5 and send him an invitation to my penthouse all the time. However, we do not know how to accept this invitation. Is there any GTA expert who can tell me how to do it? (Play on Xbox if that's important)

Normally he should get the invitation sent to the cell phone, just look through the mails and invitations

OK thanks

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