Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to apologize for missing a volunteer shift, use this factual base: identify the role, commitment, shared resource, or decision affected and who had to cover the gap.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Complete the outstanding task, replace what was lost, or accept a fair change in responsibility. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not blame the group, demand public absolution, or use a private apology to avoid team accountability. Report bullying, discrimination, hazing, threats, and safeguarding concerns through the responsible organization. You do not need the other person to agree with a boundary for you to follow it consistently.