We are using Nintex workflow with Sharepoint Online. I need to send a calendar appointment to internal users, how do I achieve this?
Also, how can I schedule workflows using Nintex Online, I don't see any options to do this.
You can create calendar appointments in Exchange using Nintex Workflow for Office 365 Enterprise Edition. If you look at the NIntex Store -> User Interaction, you will find the pack there, but you need to ensure you have Enterprise Edition.
I don't have much experience with Exchange Online, there might be web services you can call. But upgrading to Enterprise might be a more efficient way and you dont have to fiddle around with web service calls and XML transformation.
If yiu want to go with the Webservice calls you might want to look at the graph api - especially at http://graph.microsoft.io/docs/api-reference/v1.0/api/calendar_post_calendarview