Praise be to Allah.
What we say about interacting with the Shi’ah depends on the situation. The innovated beliefs of the Shi’ah vary. If it is something that does not put them beyond the pale of Islam, but is rather regarded as drifting away from the right path, such as their claiming to be devoted to Ahl al-Bayt (the family of the Prophet (peace and blessings of Allaah be upon him)), then it is permissible to greet them first, because they are Muslims who have committed acts of innovation and sin that do not put them beyond the pale of Islam, and we have to advise them and direct them towards the Sunnah and the truth, and warn them against innovation and sin. If they follow right guidance and accept advice, then praise be to Allaah, for this is what we want. But if they persist in following innovation, then they should be forsaken until they repent to Allaah and give up their innovations and evil ways, because this is a kind of punishment for them. If something good can be achieved by means of this forsaking, or something bad warded off, then it is prescribed in sharee’ah, but if this forsaking will result in something that will increase the evil caused by their innovations, then it is not prescribed.
If you think that not forsaking them will serve a greater interest and that mixing with them and advising them is more useful in opening their hearts to true religion, then there is nothing wrong with not forsaking them, because the aim behind forsaking them is to direct them to the right way and to make them feel that we do not approve of their ways, so that they may come back to true Islam.
If forsaking them will harm the Muslims’ interests and make them cling more firmly to their false ways and put them off the truth, then it is better not to do that, just as the Prophet (peace and blessings of Allaah be upon him) did not forsake ‘Abd-Allaah ibn Ubayy ibn Salool, the leader of the hypocrites, because not forsaking him was more in the interests of the Muslims.
But if their bid’ah constitutes kufr, such as cursing the Sahaabah and exaggerating about ‘Ali and Faatimah and al-Hasan and al-Husayn (may Allaah be pleased with them), and they pray to them and seek their help and ask them for support and so on, or their belief that they have knowledge of the unseen etc, which means that they are beyond the pale of Islam, then in this case it is not permissible to greet them first or to befriend them or to eat meat slaughtered by them. Rather we must hate them and disavow ourselves of them, until they believe in Allaah alone, because in this case they are kaafirs and apostates. See Majmoo’ Fataawa Shaykh al-Islam, 28/216-217; Majmoo’ Fataawa Ibn Baaz, 4/262-263
It should be noted here that it is not permissible to greet a kaafir first in general terms, such as saying Ahlan wa sahlan (welcome) and so on, because that involves honouring them and venerating them, and the Muslim is higher in status before Allaah, so we should not greet them first. But if they say that to us then we may greet them in the same manner as they greeted us, because Islam is the religion of justice that came to give each person his rights.
Al-Majmoo’ al-Thameen min Fataawa Ibn ‘Uthaymeen, 1/48
See also question no. 10843
And Allaah is the source of strength.