How can I include a Users picker in a custom field type? I would like to include a Users picker in a custom field type. Not sure if this is possible and practical.

Do you know how I can accomplish this, or can you point me in the right direction for self-education?

I see there is no users macro in /app/templates/_includes/forms.html, /app/fieldtypes/UsersFieldType.php has almost nothing in it, and I don't know enough about the system yet to know where else to look for ideas.
I don't know your requirements, but you could build a "Users Picker" with the Dynamic Fields plugin. You would basically need to do the same in its settings to build your select options as you'd do in a custom plugin.

You get your users with craft.users and then build an array with "label" and "value" keys using the merge filter.

Pass that array into the Dynamic Fields' settings or build the select options with the forms.selectField macro in your own plugin.

{% set users = craft.users.group('clients') %}

{% set userOptions = [] %}
{% for user in users %}
    {% set userOptions = userOptions|merge([{ label: user.friendlyName, value: user.id }]) %}
{% endfor %}

{{ forms.selectField({
    label: 'User'|t,
    id: id,
    name: name,
    options: userOptions,
    value: value ? value
}) }}