All member associations have membership fees, some of them have uniform fee for all members, while others differentiate it according to the status of the member (students, retired, associate, full member). Membership fees vary considerably from country to country, from around 20 to 400 EUR.
Linking membership to some kind of services or benefits helps motivating members to pay. A simple info sheet about such benefits can be helpful. Periodical reminders through newsletters, mailing lists or social media are also a useful tool to remind people. In some cases kind personal reminders may be necessary.