There are several benefits of associating your user account with your extension(s). Several being the added ability to download contacts from your portal directly to your phone, seeing voice mails for your extension right from the main portal dashboard and several others.
First you will want to log into your portal at https://my.voiply.com.
Next, click on Services>extensions. Then click on the extension you would like associated with your portal account.
Now, under the "User list" choose the account you would like associated with that extension, then click "Add".
After that, click save in the upper right hand corner and you will be completed.
If you need to create more accounts/logins for the portal, please see this article: http://support.voiply.com/knowledge-base/setting-up-a-new-portal-user