Although formal membership of the IGU is by country, individual affiliation is possible via one of its many Commissions and Task Forces. Joining them is actually very easy; it is free and it is without any commitment. The IGU now has more than 40 ‘Commissions’ (and three Task Forces) – these are groupings of geographers who are interested in particular aspects of the discipline. IGU Commissions and Task Forces cover the whole range of themes that constitute the discipline of Geography. Each of these has a Chair and a Steering Committee and organises conferences and workshops (often around major IGU events), distributes newsletters containing information related to the topic including of course the news of any upcoming meetings, and publishes books and special issues of journals etc. related to the topic. To become a member of an IGU Commission or Task Force simply write an email to the chair of the relevant Commission or Task Force and let them know you would like to become a member and be put on the mailing list for updates etc.
You can find a full list of all the IGU Commissions and Task Forces, with details of contact persons and website addresses, here: https://igu-online.org/organization/commissions/#