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 reconnect with an old support group, use this factual base: name the group, your earlier role, why you left, and the current point of contact you found.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask whether rejoining or visiting is appropriate and follow the group's present process. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not claim an old role, skip current rules, or pressure a volunteer to resolve historic conflict privately. Use safeguarding and formal leadership channels for discrimination, abuse, threats, or vulnerable members. You do not need the other person to agree with a boundary for you to follow it consistently.