VMs not allocated

Hi,

I'm building a lab for our networking module, and I've configured synchronization of users with a group. I added a handful of users to test, and under "Virtual machine pool" those users have a VM allocated and their status is "Registered".

I've now added all the users to the group, but the new users show as "Not registered" and no VM gets allocated. I thought that using sync to group meant users didn't need to be invited, and they could just login and get started? The total users is just over 200, so well within the 400 user limit. 

Is there a limit on how many users can be added to the group in one go?

Thanks,

Dean