Are you trying to launch as a guide or as a seeker? If as a seeker, you both need to have nighfall tickets. Launching as a guide does not require tickets, but does require all guides to have unlocked the nightfall on their character. [EDIT] IIRC, you can't launch as seekers together as it would mean there's only room for one guide. Is there an error message onscreen and is either your or your clan mate's emblem blinking? The blinking means that player does not meet a specific criteria, which would be indicated in an error message.