Include the sponsor/accountable owner, day-to-day process owner and anyone needed to agree scope or governance. Avoid attendees with no decision or delivery role.
For a short diagnostic, a concise weekly update is usually enough if blockers are escalated immediately. Include status, progress, next steps, decisions, risks and client actions.
Explain how the missing perspective affects confidence or timing. Replace the participant, move the milestone or proceed with the limitation stated explicitly.
Escalate when it threatens scope, timeline, quality, security, commercial terms or the intended client decision and cannot be resolved by the working team.