Members can show their interest to attend a meet-up by clicking "Attend this meet-up" button.
In addition, since Local Groups events are open to all, non-members can also attend these
Members might be bringing their non-member friends to a Local Group meet-up (and you're
welcome to do the same), but their non-member friends will not be shown as "attending"
because this feature is only available to members.
P.S.: Note that the number of attendees is not an indication of its value! Small meet-ups
work perfectly, and can facilitate deeper, more fruitful conversations.