The export is a ZIP of your group that a self-hosted MeetAgain can import. The group's own content always goes along; what goes along about each member is up to that member.
Export in the group settings shows exactly what the archive will hold before you download anything. The bar at the top counts the members who are fully included, partly included and left out. Below it, one table lists every member with what they allowed, and a second counts the rows of each kind of content - events, pages, venues, comments, catalog entries.
Ask members for permission
Sends every member who has not allowed everything a request to review their Export permissions page. Every switch there starts at No, and each member decides alone. You can ask once every 30 days.
Press Download export
The ZIP is built and downloaded. Every member included in it is told by email and notification that you exported the group.
Import it if you ever move
A self-hosted MeetAgain takes the archive under Admin, System, Import. The core is open source, so a self-hosted instance is a real option rather than a theoretical one.
Ask weeks before you need it
Members answer at their own pace. If you ever plan to move the group, send the request well ahead, so the export on the day carries everyone who wanted to come along. For a copy of your own content, the download works at any time.