Is it permissible to pray in congregation behind someone praying alone who did not intend to lead?

It is permissible for a person praying alone to intend to lead the prayer while performing it, in order to lead someone who joins him. This is supported by the action of the Prophet, peace and blessings be upon him, when he began his prayer alone and then led Ibn Abbas in prayer. Similarly, this occurred with the Prophet and Jabir and Jabbar. It is legislated for a person praying alone to intend to lead the prayer when one or more individuals join him. This is valid for both obligatory and supererogatory prayers, because the default is that they are equal in rulings unless evidence specifies otherwise.