You should be able to locate this information in your original email or invite that will have been sent to you from productsupport@actuateglobal.com
The invitation should also be showing in the diary that you have linked to your email, the joining link will be in this invitation.
Should you not be able to find this, please contact clientsuccess@actuateglobal.com and a member of our team will look into that for you and re-send your invite.