Do you add leads to your CRM via the Zoho CRM API? If so, you'll note that lead assignment rules don't work. I have developed a CRM extension called Advanced Round Robin that you can buy on the Zoho Marketplace. If you'd prefer to do it yourself, the below code can allow you to add round robin lead assignment functionality (though it's missing many of the more powerful features of the Advanced Round Robin extension). You'll need to set a few config variables like your authtoken and the user role you want leads to be assigned to. You'll also need to create a reference lead that's assigned to one of the users who should receive leads. The script will use this to figure out who should get the next lead. Thanks to Prakash from Zoho for giving me the base code for this script.
Want more options? I have a paid version of this script that you can buy on the Zoho Marketplace.