Unfortunately, the mobile apps aren't accessible by guest users, since they only have temporary accounts in order to chat in the specific room they've been invited to. Without a full-fledged account (with an email/password) the mobile apps aren't usable.
We do hope to add support for a more "permanent" level of guest user in the future, but it's not on our current roadmap. There are a few suggestions on our public JIRA (for example https://jira.atlassian.com/browse/HCPUB-1425) - you can go there to vote / watch / comment on them in order to be notified when progress happens.
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