OpenPGP solutions for all operating systems
You are using GPG Suite on macOS but your friends use Windows, Linux or a mobile operating system. Can you still securely communicate with them?
Yes you can. The requirement for that is, that they install an OpenPGP software for the platform they are using. Here are links to OpenPGP solutions for all common platforms:
You probably will find, that most of your friends do not yet have an OpenPGP key, which you need to communicate securely with them. This is an evolving technology so lets spread the word and ask them to create one. Often times people are reluctant to learn a new technology because of the initial hurdle to take. But the longer you use OpenPGP the easier it gets. Offering help with the setup can work wonders.