The game looks to match you with similarly skilled players (renown and MMR) who are located close to you and connecting to the same data centre. If you are not matched within a certain time frame, these parameters will be gradually relaxed until a match is made.
Data centre selection is based on your location. Generally, you will be connected to the data centre closest to you. If you are playing in a group with friends, the group will connect to the same data centre.
Which data centre your group joins is decided either by the location of the group's majority, or, in the case of an even split, the location of the group's leader. If the split shifts when a group member disconnects, the data centre selection will be adjusted when the group queues again.
Players who leave a match are penalised with a 10 minute ban and a loss on their record. They will also lose their match rewards and Hero progression.
Let us know if you have any questions!