Four gunmen burst into a party held at a house in Samaniego on Saturday and shot dead seven men and one woman.
The victims were part of a group of about 20 revellers.
The government of Colombia has offered a reward of 200m pesos for information about the killing in south-western Nariño province.
The motive for the attack is not clear but there are a number of drug gangs active in the border area with Ecuador.
Survivors of the attack told El Tiempo newspaper that some of them had received threats on Saturday warning them to stop the party they were attending on the outskirts of the town of Samaniego.
The group said they had decided to organise the party after an annual music festival was cancelled due to the coronavirus pandemic.
They gathered at the empty house of the grandmother of one of the organisers to drink beer and listen to music, according to the father of one of the victims.
Some received messages over WhatsApp telling them to stop and minutes later, four armed men burst onto the property on motorbikes and opened fire.
Most of those killed had either just graduated from high school or were at university.