Some organizers opt to have member dues for their Meetup group.
Member dues may be used to cover costs associated with the Meetup -- things like supplies for a project, rental of a space, or refreshments. Organizers may also use member dues to help offset the organizer subscription cost.
If you're an organizer, it is entirely up to you whether or not to collect dues and how to use them.
Please note that Meetup is not involved in the setting, collection, distribution, or management of these funds. They are a financial arrangement strictly between an organizer and their members.