I've seen the other posting about guests not having access to conversations and understand the reasons.
However this article by MS contradicts the position that guest can NOT participate in conversations.
https://support.office.com/en-us/article/adding-guests-to-office-365-groups-bfc7a840-868f-4fd6-a390-...
I think it is likely the article is misleading in that these functions are achieved thru email and not directly in the conversation view in the web portal.
We've tested and experienced what everyone else has seen with guests not having access to conversations but seeking confirmation in light of this confusing article.
