A Phrasal Verb a Day - Learn English Phrasal Verbs with Luke Thompson
#103 - TO OPT IN: = to choose to be involved in something
"We're setting up a new space project which involves a trip to Mars. If you have #2,000,000,000 you can opt in, if you want".
Also: Opt into
"By opting into this program, you agree never to return to earth again"...