I use the Custom Wizard plugin for this smooth onboarding use-case. It’s not part of the core and its an unofficial plugin but it is well maintained by the nice people at pavilion. So if you are self-hosting, then you could install it and have users fill out their interest from a dropdown and add then have the wizard automatically add them to the related groups and/or update their profile fields to include some of that information. Lots of other options, but worth looking into if you are able to install your own plugins.
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