How to modify entry with a users field in the front-end? Is it possible to modify an field of an entry within the front-end? Something like this with an action URL:
&lt;a href="{{actionUrl('save/entry', {userfield:user.id}) }}"&gt;add user to userfield of current entry&lt;/a&gt;
Sure... you'd just do it through whatever input tag you want using the name="fields[fieldHandle][]" syntax passing in the user's ID as the value.

There are examples of that here, under the "Relations" header:

https://straightupcraft.com/articles/craft-cms-field-guide-twig?view=input

You won't be able to do that should an anchor link GET request though... EntriesController::saveEntry() is explicitly checking for a POST request, so you'll want to do it though a proper form: https://craftcms.com/docs/templating/entry-form