In the Name of Allaah, and all praise is due to Allaah, to proceed;
Jama’atul-Tableegh do not have with them sure-sighted knowledge in issues of ‘Aqeedah, so it is not permissible to go out with them, except for the one who has with him knowledge and insight of the correct sound ‘Aqeedah, that which the Ahlus sunnah wal-Jama’ah are upon, so that he guides them and advises them and cooperates with them upon good, because they are very active in their work, however they are in need of more knowledge, and (in need of) those who will give them clear-sightedness from the scholars of Tawheed and Sunnah.
May Allaah grant all people understanding in the religion and firmness upon it.
As for the Prayer in Masjids that contain a grave within them, then it is not correct, and it is obligatory to repeat that which you prayed within them; due to the saying of the Prophet sallAllaahu ‘alayhi wa sallam:
لعن الله اليهود والنصارى اتخذوا قبور أنبيائهم مساجد
"May the curse of Allaah be upon the Jews and the Christians, they took the graves of their Prophets as places of worship." Its authenticity is agreed upon.
And his sallAllaahu ‘alayhi wa sallam saying:
ألا و إن من كان قبلكم كانوا يتخذون قبور أنبياهم وصالحيهم مساجد، ألا فلا تتخذوا القبور مساجد فإِني أنهاكم عن ذلك
"Truly those that came before you use to take the graves of their Prophets and righteous ones as places of worship, so do not take the graves as places of worship because, indeed I forbid you from that."
And the narrations in this subject are many.
And success is with Allaah, and may the peace and blessings of Allaah be upon our Prophet Muhammad, and upon his family and Companions.
 Reported by Muslim in his Saheeh.