Nintex Forms Group Page

but once a form has been submitted

you do not submit form, you submit list item or document to a library.

 where would users (ideally a group) go to view their open forms? 

so you have to see/open respective items in a list or library and they will open in a form. if you designed your custom form for a list/library they will open in your custom form. otherwise they will open in sharepoint's default form

 I am sending an email with the Flexi Task,

if you create a task for a (group of) user(s) then that's a different item from a completely different list (task list).

since it's a different list it might have designed completely different form (task form) independent of above mentioned (source) list form.  this form is to be used to respond to the task.

 but was wondering if the open forms could be displayed on a group/department page?

what are 'open form' in your meaning?

if you mean assigned (and not yet responded) tasks you can find them in a task list.

or you can configure 'My workflow task wepartb' where each user can see all the task assigned to him

https://help.nintex.com/en-US/nintex2013/help/#Workflow/RootCategory/SharePoint/Nintex.Workflow.ItemsPendingApproval.htm… 

if you mean to see submitted items, then as mentioned above you can see them in respective list/library.

or you can configure another 'List form webpart' into your group page.

https://help.nintex.com/en-US/nintex2013/help/#Forms/RootCategory/SharePoint/Nintex.Forms.ListFormWebpart.htm%3FTocPath%…