NodeXL can import data in multiple formats.
You can represent your data as a matrix, people along the left column, events along the top row.
See: NodeXL>Data>Import>From open matrix workbook
You can also represent your data as an "Edge List" that might look like:
Person, Event Name, Information about Person, Information about Event (like Date or # attendees)
NodeXL can import data in this format.
See: NodeXL>Data>Import>From Open Workbook
You are describing a multimodal and multiplex network.
It is multimodal because you have more than one type of entity (People, Events, Organizations).
It is multiplex because people connect to events by the relationship "Attends" and connect to organizations via the relationship "joined".
In a multplex dataset you might create an edges list with a column that describes the nature of the connection, a "Relationship Type":
Person, Event or Organization Name, Relationship Type, Information about Person, Information about Event or Organization (like Date or # attendees)