(intr, adverb) to choose to be involved in or part of a scheme, etc
A Brief History Of Writing InstrumentsThe quill fueled most written communication from the 6th to the 19th century. Quills were made of hollow bird feathers, most commonly goose feathers.
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Examples from the Web for opt in
Lulu, the mobile dating app for women, has changed its system to allow men to opt-in.
Provide an opt-in system for religious organisation who wish to conduct marriages for same-sex couples.