My parents had me practice going up to kids to ask them if they wanted to play. In the process, I got used to rejection but also enjoyed making new friends easily. They started really young, so I never really experienced it as a difficult thing to do, although it didn't come naturally to me (it didn't occur to me to do it by myself).
Just teach her that 99% of rejection she will experience in life isn't actually about her (or at least not solely about her), but is a completely normal part of life. That's not to say she should work for her accomplishments, etc. of course.
And of course love her and support her, and don't ever put her down or make her feel like she is less than adequate.