We would like to understand how many users come on each week grouped by source. We have a required question on onboarding that asks for referral source, but it happens after our first paywall. Is there a way to group our trial start event, which could occur before the user property is set, by the “how did you hear about us?” property which is set later?
Yes, you can use the “Group Segment by” feature. This will show the users’ current value for referral source, even if it was null at the time the Start Trial event fired.

I have that set, but it’s explicitly saying none for users who do not have those properties set:

https://amplitude.com/docs/faq/unexpected-values-in-user-counts It seems like according to this chart, it won’t group them by this property if it’s not set at that time.
Yep, I just stumbled on that article myself and stand corrected. Can you use Any Active Event broken down by that property instead?
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