Give the complete event context
State who is organizing, the group, date, location, expected size, cost, accessibility, and how contact information was found. Name who will attend only when you have permission.
Acknowledge a complicated past when relevant
If there was bullying, exclusion, conflict, or a painful ending, do not market the event as automatically fun for everyone. A brief acknowledgment makes the invitation more honest.
Remove the cost of saying no
Say that declining or not replying is completely fine and do not recruit others to persuade them. Protect their contact details from group lists unless they opt in.