Is it permissible to join a religious institution that undertakes blessed charitable and da‘wah (proselytizing) work, but whose contemporary legal reasoning (ijtihad) in modern issues is not entirely trusted due to its divergence from the opinions of the majority of scholars?

There is no harm in cooperating with religious institutions in acts of goodness, due to the Almighty’s saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." If they have erroneous interpretations, do not follow them in those, and ask scholars, and supplicate to Allah to guide you to the truth, as the Prophet (peace be upon him) used to supplicate: "O Allah, Lord of Jibril, Mika'il, and Israfil, Creator of the heavens and the earth, Knower of the seen and the unseen, You judge between Your servants concerning that over which they used to differ. Guide me to the truth concerning that over which they differed, by Your permission. Verily, You guide whom You will to a straight path." Even if you ascertain their error, that should not prevent you from cooperating with them in what they are right about concerning good. So, if people do good, do good with them, and if they do evil, avoid their evil.