Start with the part that is verifiable
Separate the event you can describe from the motive or reaction you are guessing. For a message about how to reconnect with a former union colleague, use this factual base: name the group, your earlier role, why you left, and the current point of contact you found.
Make one answerable request
Give the recipient one concrete question or next step instead of several emotional jobs at once. 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.
Leave the recipient in control
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. A clear message remains useful even when the answer is no or no answer arrives.