Shaykh Abd al-Aziz bin Baz, Rahimahullah, who was the most well wishing to the Ummah was asked,
QUESTION: Is it permissible to cooperate with the Islamic groups active in the field? To what extent is cooperation permissible? Please advise, may Allah reward you with the best!
ANSWER: It is permissible and even obligatory to cooperate with all Muslims in righteousness and piety as much as possible because Allah (Glorified and Exalted be He) says:
"Help you one another in Al-Birr and At-Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression." (Surah Al-Ma'idah, 5:2)
"By Al-'Asr (the time). Verily, man is in loss, Except those who believe and do righteous good deeds, and recommend one another to the truth, and recommend one another to patience." (Surah Al-'Asr, 103)
It is cooperating in righteousness and piety and enjoining one another to adhere to the truth and showing patience to clarify the rulings of Shari'ah (Law) and warn against Bid'ah (innovations in Islam) and to make clear the sound creed followed by the Salaf (righteous predecessors of the Ummah (nation based on one creed), the Companions of the Prophet (peace be upon him) and those who followed them in righteousness.
Cooperation and recommendation is to enjoin what is right and forbid what is wrong and to abandon those worthy to be abandoned because Allah (Glorified and Exalted be He) says:
"The believers, men and women, are Awliya' of one another; they enjoin Al-Ma'ruf , and forbid (people) from Al-Munkar ; they perform as-Salah, and give the Zakat, and obey Allaah and His Messenger. Allah will have His Mercy on them. Surely Allah is All-Mighty, All-Wise." (Surah At-Tawbah, 9:71)
"Those among the Children of Israel who disbelieved were cursed by the tongue of Dawud (David) and 'Isa (Jesus), son of Maryam (Mary). That was because they disobeyed (Allaah and the Messengers) and were ever transgressing beyond bounds. They used not to forbid one another from Al-Munkar (wrong, evil-doing, sins, polytheism, disbelief) which they committed. Vile indeed was what they used to do." (Surah Al-Ma'idah, 5: 78)
The Prophet (Sallalahu Alayhi Wasallam) said:
The similitude of believers in regard to mutual love, affection, fellow-feeling is that of one body; when any limb of it aches, the whole body aches, because of sleeplessness and fever.
The Prophet (Sallalahu Alayhi Wasallam) also said:
Islam is based on advising one another. Upon this we said: For whom? He replied: For Allah, His Book, His Messenger and for the leaders and the general Muslims. (Related by Muslim in his book of Sahih (authentic) Hadith).
There are many Ayahs and Hadiths to the same effect.
Imam Ibn Baz, al-Fatawa, 28:248.