All people who claim to be prophets after prophet Mohammad (pbuh) have used the same technique to make people follow them, Almahdi technique!
Since after Mohammad's (pbuh) death up to now, There were many cases where people claim to be Almahdi to make the people follow them and some of these actually somehow succeeded.
The question now is why do people who claim to be prophets always claim to be Almahdi? simply because Muslims know for granted that Mohammad (pbuh) is the last prophet which was clear in Quran and the only person with (prophety) attributes still to come is Almahdi beside Messiah (Jesus) who we as Muslims believe will come at the end of time. This was mentioned in a lot of Hadiths said by Mohammad (pbuh).
The leader of Ahmadiyya (Mirza Ghulam Ahmad) claimed to be both Almahdi and Messiah and he used this to convince people to follow him and he actually made some followers!
Regarding him being a messenger/prophet, He claimed that Ahmad was mentioned in Quran and God meant him. While Ahmad is a known name for prophet Mohammad who has another names also:
وَإِذْ قَالَ عِيسَى ابْنُ مَرْيَمَ يَا بَنِي إِسْرَائِيلَ إِنِّي رَسُولُ اللَّهِ إِلَيْكُم مُّصَدِّقًا لِّمَا بَيْنَ يَدَيَّ مِنَ التَّوْرَاةِ وَمُبَشِّرًا بِرَسُولٍ يَأْتِي مِن بَعْدِي اسْمُهُ أَحْمَدُ فَلَمَّا جَاءَهُم بِالْبَيِّنَاتِ قَالُوا هَذَا سِحْرٌ مُّبِي
And [mention] when Jesus, the son of Mary, said, "O children of Israel, indeed I am the messenger of Allah to you confirming what came before me of the Torah and bringing good tidings of a messenger to come after me, whose name is Ahmad." But when he came to them with clear evidences, they said, "This is obvious magic."[61:6]
Ahmadiyya followers believe that their leader is Almahdi and the Messiah and the Messenger who was mentioned literally in Quran. Other Muslims believe in Almahdi and Messiah but just not Mirza Ghulam Ahmad. There are Hadiths describe Almahdi and Mirza does not fit the description. Any way the time was not clearly mentioned. The prophet (bpuh) said it will be at the End Of Time which we do not know when exactly.