The treaty was signed on January 23 by Holland, Zeeland, Utrecht (but not entire Utrecht) and the province (but not the city) of Groningen. The treaty was a reaction of the Protestant provinces to the 1579 Union of Arras (Dutch: Unie van Atrecht), in which the southern provinces (most of them now part of Wallonia) declared their support for the Catholic Spanish.
On February 4, Ghent also signed. In March some cities from Friesland joined it, as well as 3 of the 4 quarters of Guelders. In the summer, Amersfoort from the province of Utrecht also joined, together with Ypres, Antwerp, Breda and Brussels. In February 1580, Lier, Bruges and the surrounding area also signed the Union. The city of Groningen shifted from contra to pro under influence of the stadtholder for Friesland, George van Rennenberg, and also signed the treaty. Later on, Zutphen also signed so Guelders (of which Zutphen is one of the quarters) supported the Union completely. This happened in April 1580, as did the signing of Overijssel and Drenthe.
This leads to a general and simplified overview of the parts that joined:
Flanders was almost entirely conquered by the Spanish troops, as was half of Brabant, so they dropped out. The United Provinces still recognized Spanish rule after the Union of Utrecht. However, the Union contributed to the deterioration in the relationship between the provinces and their lord, and in 1581 the United Provinces declared their independence of the king in the Act of Abjuration.