Different user group upon joining
 

Is there any way to set criteria somehow (such as by referrer entered during registration) which will put someone who registers into a different user group (or to add a secondary {aka additional} user group or something like that) than the standard 'registered' user group?

You will have to add a plugin to do this.

Are you aware of a plugin that would do this?

I don't know if there is one readily available. You may have to code your own.