

Phone number and trust-on-first-use for most people, with out-of-band fingerprint verification for the paranoid. It really depends on the threat model and the security practices/awareness of your colleagues, but a link shared on some social media or lower-security chat network is more vulnerable to a man-in-the-middle attack than a phone number for your average Joe. There are a lot of ways a person could get a manipulated invite link.
Looks like a LOWESS curve (https://en.wikipedia.org/wiki/Local_regression). They always overfit, but are still useful to show trends. The main danger is that they get wonky near the limits of the data. Note that that increase at the end of the left plot for Democrats looks like it is increasing -but that increase looks to be 100% dependent on that single data point for '25. Obviously, you never want your analysis to be dependent on a single point like that.