Adding Groups to Enrollees

I have a problem when trying to add groups which have lots of users in to a course. It works fine for individual users or groups containing a few hundred users but we have 2 groups containing approx 3000 and 18000 users and when you try to add them you get an error message

"Operation Failed - Error Code:operation-size-error, SubCode:"

Any ideas? Alternatively if anyone has the support phone number for On Prem installations could you let me know I think I have written it down wrong on a previous occasion.

Thanks