confraternizar

To fraternize — mix warmly with people from an opposing or separate group, most often soldiers, rival teams, or coworkers from different departments. Formal/journalistic register.

• Los soldados confraternizaron durante la tregua. — The soldiers fraternized during the truce.
• La empresa organiza actividades para confraternizar entre departamentos. — The company organizes activities so departments can bond.
• Los aficionados de ambos equipos confraternizaron antes del partido. — Fans of both teams fraternized before the match.

Formal/written register — in everyday speech, llevarse bien or socializar is the more natural way to describe ordinary bonding.

• politics Tregua de Navidad de 1914 — During the WWI Christmas Truce of December 1914, German and British troops spontaneously left their trenches to exchange gifts and carols; historians describe the episode using confraternización, and military commanders explicitly banned further confraternizar for fear it would weaken fighting spirit.