Hi @grandeau!
I am sorry for the delay in getting back to you about this! Unfortunately I do not have good news as eazyBI does not allow to create a user group dimension for other than default user-picker fields. @martins.vanags added this to eazyBI backlog based on your request some time ago and I upvoted this request again, but no estimates for now.
If you are maintaining the Epic Assignee with a scripted field, maybe there is an option to get the Epic Assignee group in the same way? Scripted field does not need to be displayed in the Issue screen, but it can be imported to eazyBI as a dimension.
Lauma / support@eazybi.com