neither, for obvious reasons. You need the present participle form of the verb rather than the past participle in that particular usage. You are not employing the verb as an adjective (the stolen dog) and you are not employing passive voice (I was taken by a space alien), you are using the present continuous tense, which is declared by use of present participle. the subject(s) is/are in the process of doing an action. They are getting on the bus, they are taking the bus. They could, conceivably, be spoken of as "they were taken by the bus" (passive voice in the past) but never "they were gotten on by the bus".