Is it permissible to form an association to collect money monthly from female colleagues, then hand over the amount to one of them after deducting 400 riyals for fees and assuming responsibility with everyone's consent, or does this fall under usury?

There is no objection to establishing the aforementioned association, nor to receiving a fee for following up with subscribers, safekeeping funds, and distributing them, provided the association members agree to it. This fee is in exchange for the effort and safekeeping, not for guaranteeing the funds, as the possession here is one of trust (yad amanah), and a trustee is not liable unless there is transgression or negligence. However, taking a fee without the members' permission is a forbidden betrayal.