What is the ruling on the association that employees form at the end of the month, which helps employees with many things, including marriage?

If the intent is what employees collect from their monthly salaries to pay to one of them in rotation, then this is permissible and there is no objection to it. Sheikh Ibn Baz, may Allah have mercy on him, issued a fatwa to that effect, and the Council of the Senior Scholars, by majority, affirmed its permissibility due to the benefit it entails for everyone without harm.