I am an organisation admin. I don’t want anyone to request membership and want to manually add/remove members.

Organisation admins have the option to make the organisation an open or closed group. By default, all organisations are an open group to allow new users to request membership. If you don’t want to allow any member to join your organisation, you can turn off the ‘Allow members’ checkbox under ‘Edit organisation page’. This will make your organization a closed group with existing members. No new member will be able to send a request to join your organization on HDX. The admin(s) of your organization can still manually invite new members, remove existing members or change their roles from the ‘Members’ tab.