Eventually I found the answer at https://laracasts.com/discuss/channels/spark/adding-registration-fields-for-teams, though I can update this for Spark v9. It seems that you need to add the following function in the booted() method of SparkServiceProvider.
Spark::swap('Register@configureTeamForNewUser',
function (RegisterRequest $request, $user)
{
if ($invitation = $request->invitation()) {
Spark::interact(AddTeamMember::class, [$invitation->team, $user, $invitation->role]);
self::$team = $invitation->team;
$invitation->delete();
} elseif (Spark::onlyTeamPlans()) {
list(self::$team, $paymentId) = Spark::interact(CreateTeam::class, [
$user, [
'name' => $request->team,
'slug' => $request->team_slug,
'venue_address_line_1' => $request->venue_address_line_1,
'venue_address_line_2' => $request->venue_address_line_2,
'venue_town' => $request->venue_town,
'venue_county' => $request->venue_county,
'venue_postcode' => $request->venue_postcode,
'venue_country' => $request->venue_country,
'venue_type' => $request->venue_type,
'venue_type_other' => $request->venue_type_other,
]
]);
}
$user->currentTeam();
});