Good question! We've made it nice and easy for existing clients to signup on their own. 

When the client goes to your client portal page for the first time, 

(Example: https://demo.mykcapp.com/clientlogin) they will click the "register now" option at the bottom of the login screen. If they are an existing client they'll choose the existing client option and be presented this form:

Here's where the magic happens. To link to their already existing client record in your KC software it will require a match of:

EITHER First OR Last name

WITH

EITHER email address OR phone number. 

Example: 

Steven Smith in the above screen shot used Steven Smith. Your KC has Steve Smith. He also used a new phone number above which your KC does not have but it DOES have the same email address. 

The system will take SMITH and parse the database to match it with his email address Stevensmith@testemail.com.. This will link his login with his correct account in KC. 

Example 2: 

In your KC database you have customer:

Tara Saunders

5555555555

Tara@Testemail.com

Tara signs up for client portal as an existing customer but she just got married so she changed her last name AND changed her email address so she uses:

Tara Thompson

5555555555

TaraThompson@testemail.com

The system will take TARA and parse the database to match it with her phone number 5555555555.  This will link her login with her correct account in KC.