We DO NO ASK to sign up or request emails
There is a meeting and people are coming to a meet point. You need to know how far are they with this App you can keep track of that. Start up a GPS meet and send the invitations in the form of an email, the attendees get an email with links to Join the meeting and to install the App if they don't have it yet.
You do need Data access, and users running the Android App,an iPhone version is coming soon so mixed meetings can be carried out.
Option to email you coordinates recipient gets a link that loads Google maps with your position on any device/OS that has access to Google maps.
You may also keep track of vehicles on the road and so many other possibilities.