My ferry rule: Never book in advance and never buy round trip. We always walk up and take the next boat leaving. Very seldom are the ferries at capacity. That said, your late group should have no problem making Speedy's 4PM to VG even wiht a little delay in arriving. Speedy is by far the most reliable ferry but if the other party is delayed to long none of the other companies will give credit for a Speedy's ticket.

Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. - Mark Twain