On United Airlines, flyers can smoothly board their flights by ignoring the crowd at the airport gate. This is because this airline offers different boarding groups to passengers based on their fare class and status. Moreover, for continental flights, Boarding Groups United are divided into six segments, which are explained in the points below.

This includes pre-boarding.
Group 1 and Group 2.
Group 3 and Group 4.
Group 5 and Group 6.
For more information regarding the boarding groups of United Airlines, flyers can read the blog post outlined by Group Travel Airlines. 
Read More: https://grouptravelairline.wixsite.com/flights/post/united-airlines-boarding-groups-a-complete-guide