Problem inviting a colleague

Problem inviting a colleague

I have set up my free version of Zoho CRM and have also set up, activated and invited a colleague. Although the system tells me that an invitation has been sent to my colleague they never land with him.
 
I have tried this on a number of occassions but as yet no luck.
 
What other way might I provide my colleague with a username and password to enable him to login to my system?
 
Thanks for your help
Steve