Hi
It appears that there is no direct way to create a "Join" button on a SharePoint page that allows users to join a Viva Engage community without first navigating to the community page. The Viva Engage Web Part in SharePoint primarily provides a "View All" button, which directs users to the community page where they can then click "Join."
Personally, I completely agree with you, this is indeed a good addition to the current features of SharePoint. Thank you for bringing things to our attention. We sincerely recommend you go to SharePoint Feedback community to provide your feedback: SharePoint · Community (microsoft.com). This is the best platform to let related team hear from you and make the products and services better for you and others.
Thanks.
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