>>33754160This. Then, one-by-one, far from one-another's help, they each get captured and jailed deep in their dungeons.
Until only one is left free.
This one must free another to help him three a third, and so on until all are freed. Without implicit instruction by the GM the idea must naturally enter the minds of the players that participation and cooperation by all free players is required to free each captured character. Eventually, as the last character is freed, they learn the value of teamwork. Hopefully, they stick together? Probably not. Adapt.