I have Alayen, Ymira, Nizar, Matheld, Artimenner, Borcha, Rolf, Baheshtur, Bunduk, Katrin, and Marnid in my party and nobody has left yet. I've seen their morale dip to as low as 35 but no one has deserted yet. My persuasion skill is at 4. The key is to try to have 1 person they hate and 1 person they like in the party to balance it out. Some people seem to hate others more than others though. Matheld hates Ymira alot and always has the lowest morale, while I found that marnid has 2 people he hates in the party and his morale is always high. I expected Marnid to desert after I found everyone but it seems unlikely to ever happen given his morale never dips. It's around day 280 right now.