Withdrawing from project I am helping in
Withdrawing is a loss-loss situation and I will not recommend it. You have done your work and they seem to have the motivation to release it some day. If you withdraw now, the work is gone, tension arises, friendship threatened. These are not worthy for a short moment of revenge-induced pleasure.

If you feel your input is so instrumental then take this as a lesson to bargain better; in future when similar collaboration comes along, make "have the right to present at xxx conference [year/month/day]," or similar terms as your condition. You should also raise the issue of authorship from the get-go.

Ascending one more level, if you're indeed that much more experienced than the team in terms of how the research realm works, why not help them further? Connect them with this very meeting that you plan to attend, and propel the work forward by letting more people know about it.