Unable to add more than 64 CICS Resource Groups in a Startup List.  When trying to add a 65th Group, message says "Update Successful", but the Entry Count remains at 64 and the new group is not added.

Unfortunately this is a hard limit in the product.  You can only add up to 64 groups in a startup list.  When you go into the details of a Startup List and click on "Help This Page" (on the bottom left hand side of the page), you will see that this is a documented limit:

01 - nn
Lists the resource groups in the startup list.
A startup list can reference up to 64 resource groups.