A group needs someone to organise the next event. You care about the cause and offer to help. The role may include booking a venue, answering questions and cleaning up afterwards, even if only one of those jobs appeared in the invitation.
Ask what the role includes and how long it is expected to last. Choose the part you can do alongside your existing responsibilities. Say if you can cover one event rather than an ongoing position.
Make the workload shareable
List the tasks with the organiser and identify which need other owners. If nobody can take an essential task, the group may need to simplify the event. Leaving that task unnamed does not make it disappear.
Keep a note of the arrangement and review it when plans change. You can care about the group's work while having limited hours available. If you need to step back, give notice and hand over the information another volunteer would need. Avoid offering a replacement date unless you expect to honour it.
