Trying to figure out how to implement the solution outlined HERE Put quite a few hours into using a secondary registration list that uses the Title as a Lookup to the Calendar Event Title. That's all well and good but for the life of me can't get the work flow to pipe in the registrant to the "Attendees" field. The best I got was it wiped out what was in there and replaced with the "current user". Tried many variations of fashioning a variable out of the existing users in that field and add ;[WorkFlow Context:Initiator] to it but doesn't work. Then started looking at the SPUser code and tried with VS 2012. After much struggle getting VS 2012 to connect to the SharePoint 2013 site it throws a million errors on the deployment. I basically added a visual web part and threw two buttons in there- Register & Unregister. On the Register button I added your code to the Click session but I am sure I am missing something (many errors like 'SPUser' is a type and cannot be used as an expressing, 'user' is not declared, 'SPFieldUserVAlueCollection' is a type and cannot be used as an expression...).
If you can offer any help I'm not a coder so this one is tough for me- Or if there is a way I am missing to add another field to the Calendar list and have it stored as an array perhaps I can accomplish it that way (don't think its possible)
Also tried setting an email workflow to match the "Created By" field on this secondary list where the Title matches the Event name. This would also meet the requirement where the calendar list can trigger a workflow based on "item changed" and send to anyone who resgistered