How is love for the sake of Allah? And how will a person be resurrected with those he loves?

Love for the sake of Allah means that you love a person because of their uprightness in obeying Allah and their avoidance of sins. As for the meaning of "A person will be gathered with those whom he loves," it means that Allah Almighty will gather him on the Day of Resurrection with those he loved in one place, as stated in the verse: "And whoever obeys Allah and the Messenger - those will be with those upon whom Allah has bestowed favor of the prophets, the steadfast affirmers of truth, the martyrs and the righteous. And excellent are those as companions." This does not necessarily mean that his status and reward will be exactly like those he loved in every respect.