Many Shopify themes come with a basic newsletter sign up form in the footer or right column, but it is not connected to the Shopify backend. I.E - It does nothing when you click submit.
You can easily fix this in your themes. When properly connected, a newsletter sign up will be fed to HubSpot via Hubshop.ly and show as a conversion event.
Replace your email signup form with this code:
<input type="hidden" id="contact_tags" name="contact[tags]" value="prospect,newsletter"/>
<input type="hidden" id="newsletter-first-name" name="contact[first_name]" value="Newsletter" />
<input type="hidden" id="newsletter-last-name" name="contact[last_name]" value="Subscriber" />
<table>
<tr>
<td>Enter in your email to join our mailing list:</td>
<td>
<input type="text" id="contact_email" name="contact[email]"/>
</td>
</tr>
<tr>
<td colspan="2"><input type='submit' class="submit" /></td>
</tr>
</table>