You cannot use acf_form() for this. The reason is that you want to have a single form that updates a field but multiple user. Well, you may be able to use acf_form(), but either way you need a custom form for this page.
There are many steps. Basically to use acf_form() you would need to create a field group that is used only on this page that has the repeater. You would need to create a filter for the repeater field, I’m not sure right now if it should be acf/load_value or acf/prepare field.
The you would need to create an acf/pre_save_post filter that reads through the submitted repeater field and updates each user accordingly and then somehow prevents ACF from updating the values. Alternatively to the pre_save_post filter you could set up the acf form to save the values to an options page and then use an acf/save_post filter to read through the repeater and update the user fields.
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