Update How To Engage PMs on Customer Calls

Why is this change being made?

PMs have shared feedback that they're not receiving context prior to customer calls. This change adds guidance for the field team on how to engage with PMs when adding them to customer calls. Includes a link to a new briefing document template that the field team can use to provide context. The template I proposed is a shortened version of the template used for exec prep before customer calls. Feedback very welcome!

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