Whether religious or lore oriented, pilgrimages can be an effective way of compiling a party of adventurers. Characters may be traveling to a holy place or a site renown for some significant event. You never know who you might bump into or what might occur along the way or at the pilgrimages end. Attending the funeral of a friend brings many people together - some for the very first time. Added to this, if the death were for some reason other than natural, young brash adventurers might take it upon themselves to avenge the wrongful death of their friend.
Death can be a strong motivator in aligning characters for the common cause of revenge.
This can be a tough one to work with, but it can be done. In feudal societies, the common setting for many fantasy RPGs, war and conflict were a common occurrence. In times of need, many were willing to pick up sword or staff to defend their homeland - from all across the kingdom. Adventurers are all the more likely to travel forth in search of the glory of battle. The problem with this scenario is that the characters are likely to become stuck in a war and unable to set off on adventure without certain risks.