The content of the welcome emails are found in our language file /language/en-GB/en_GB.com_jfbconnect.ini. Search for the key "COM_JFBCONNECT_EMAIL_REGISTERED_BODY" to find the area for the appropriate strings
The subject is in the com_users language file, /language/en-GB/en_GB.com_users.ini. Search for COM_USERS_EMAIL_ACCOUNT_DETAILS.
-Melissa