With Teamup Calendar, you can enable signups for any event (or by default for all events on the calendar). Anyone with access to the calendar event can then sign up with their name and email address.
If someone includes their email address when they signup for an event, they will receive an email confirmation.
The confirmation email includes the event details, a link to the event, and an attached ICS event file. The event file can be used to add the Teamup event to another calendar.
With the latest update, all popular email clients like GMail and Outlook will now prompt the user with an easy interface to add the event to their own calendar.
If an email client does not include the buttons, it's still easy to open or download the ICS file to add it to another calendar.