If you want to be center stage and lead act of the show, you cannot sing backup. The meaning of singing backup is to sing behind someone, not compete with them. If you can control your need to be the center of attention, you can learn backup singing.
Backup singing is more difficult than you might imagine. Similar to an instrumental accompanist, this method of singing requires that you listen very intently and be responsive . Listening and reacting to the main singers phrasing, you must begin and end as they do. It may well be good to think of yourself as the singers shadow, never noticed but always present.
Hopefully you have some input or control over where you sit or stand on stage. See to it that you can see the singer at all times. If you cannot see the face of the singer your job will be difficult indeed. Watch the singer to learn where they are going. If you are watching the singer you can see when they make a mistake or intentionally take another repeat etc. Your eyes must be glued to the singer. Often a singer may use a gesture such as a nod of the head to indicate where they are going. Learn their motions and watch out.
You must blend your line to the singers phrase. If you want to blend in the proper way, start your line just after the singer starts theirs. If you are not sure of how loud they are going to be, start your melody quietly and raise your level as the phrase develops. Another tip is to be easy on the consonants at the start of a line. Two consonants sung slightly after each other will sound disagreeable and is largely avoidable.
The same technique is to be applied to the ending of all melodies as well. As the singer approaches the end of a melody, gradually decrease your volume so you can avoid the embarrassment of ending after they do. If you follow these singing tips, watch, listen and blend in; you can be a professional backup singer in very short order.