Pick a build, install on a registered device, test, file issues. Android installs APKs directly; iOS needs device registration (see below).
iOS installs require device registration.
iOS builds are signed with our Apple Developer provisioning profile, which limits installation to a whitelist of devices. To get added, send your device UDID plus a short cover letter (who you are, what you'll test) to lead@onym.app. Android testers can skip this -- APKs install on any device with installs-from-unknown-sources enabled.