Is it better for a solitary person praying Asr for two people to line up behind him, or for the two to start a new congregation? And does the solitary person receive the reward of congregational prayer if people line up behind him after the opening takbir?

It is permissible for someone who enters the mosque after the congregational prayer has concluded and finds an individual praying alone, to join him in prayer to gain the reward of congregational prayer. The individual who becomes an imam also receives the reward for this. Changing the intention from praying alone to leading the prayer does not cause any harm. This is the fatwa issued by Sheikh Ibn Baz and the scholars of the Permanent Committee, based on the hadith: "Who will donate to this one by praying with him?" However, it is preferable, if the person entering finds another person who has not prayed, for them to form a congregation together, in order to avoid scholarly disagreement.