rcp_edit_subscription_form

Used to inject content into the "Edit Subscription Level" table. This can be used to add a new field to the "Edit Subscription Level" form.

For saving the custom value, you'll want to use the  rcp_edit_subscription_level action.
Example:
function rcpga_edit_groups_checkbox_subscription_form() {
	$seat_count = ( empty( $level->id ) ) ? 0 : rcpga_get_level_group_seats_allowed( $level->id ); ?>
	<tr class="form-field">
		<th scope="row" valign="top">
			<label for="rcpga-group-seats"><?php _e( 'Group Seats', 'rcp-group-accounts' ); ?></label>
		</th>
		<td>
			<input id="rcpga-group-seats" type="number" name="rcpga-group-seats" value="<?php echo absint( $seat_count ); ?>" min="0" style="width: 100px;"/>
			<p class="description"><?php _e( 'The number of group seats available to this level including the group owner.', 'rcp-group-accounts' ); ?></p>
		</td>
	</tr>
	<?php
}
add_action( 'rcp_edit_subscription_form', 'rcpga_edit_groups_checkbox_subscription_form' );