There several ways to merge a lead with a user.

1. Manual merging (CSV import) - When you have a lead list and just want to import it in Intercom and merge with existing customers or with a customer who will open an account later.

To manually import leads and merge with a user go to:

Contacts -> All users -> "New users or leads" -> Import People -> CSV (Launch Importet) and new window pops up.

Switch to import leads and upload CSV File.

More information about lead data import can be found here

2. Automatically merge profiles

Once non-login users chat with you they have lead status, once this user makes authorization to your app - lead changes to the user.

Example: If I started a conversation at http://homepage.comIntercom will show me as a lead. 

Once I go to HTTP://homepage.com/app/login and make authorization I will become a user and this conversation will be attached to the user.

Sometimes there cases, where customers (as a lead status) ask support agents some requests that require user verification. In this case, your support agent need's to ask the customer to go to the login page and make an authorization for further assistance (once the customer makes authorization, it will display as a user). 

Off-topic: If you are good with development, you can develop a log-in app and ask customers to make authorization directly in Intercom messenger.

Keep in mind that for automatic merging you should already have been set-up Intercom for logged-in users. More information can be found here.

3. Merge with API

You can read more information about merging here.

Here's an easy APIANT template to merge leads with users. Just add "lead:Id" as a conversation note in the user and they will merge instantly!

https://apiant.com/templates?template=cab19a0f26cd431e8a948dc280689c2d/p>