How can I 'Parse' { entry.belongsTo } to array with users?
You should be able to use the full Twig tag for what you want to do.  Perhaps something like the following (which assumes you have an email field you are trying to output for each user and want to separate your emails by a comma):

{% for person in object.belongsTo %}{{ person.email }}{% if not loop.last %},{% endif %}{% endfor %}


renderObjectTemplate should parse both the abbreviated syntax with a single curly brace or complete Twig tags.  To take a close look, the abbreviated syntax:

{ belongsTo.first.fullName }


Is being interpreted by Craft as the following:

{{ object.belongsTo.first.fullName }}