I'd like to join this group too; apologies to Vojtech and Christian: it took me long to respond to this thread.
I agree that withdrawal and enrollment are useful information to be included about retention status.
I'd like to add visit_ID because recurring visits are common in clinical trials; visit_ID can help refer to a specific visit in the protocol and link with study calendar, adverse events, etc.
Another relevant field is adverse events.
What is the scope of this project? Does this cover only completed trials? How about supporting pragmatic trials? if it is the latter, we need to add more data fields.
Could we plan for a f2f discussion during the upcoming OHDSI meeting for this group? I will be there 10/17-19.