For some kids, exposing themselves to a group of others is difficult. I can imagine your daugther is afraid of being rejected or ridiculed by the group in public, and might need a bit of help to overcome this fear at first.
You could step in, walk over to her friends with her, say hello, make some smalltalk with the girls, get your daughter involved in the conversation and then pull back once she's settled in.
Then once you did this a few times, just prompt your daughter to try it herself, possibly suggesting a few things she could say to melt the ice when walking up to the group.
This way, your daughter gets to see how you make contact with her friends, she sees that nothing bad happens when you do, and she can copy your example.
You could also talk to her about how she can react to bungled social interactions (being able to laugh at yourself instead of being mortally ashamed is very handy, but was terribly hard for me to learn as a child), so that she has a backup plan for what to do if something goes wrong - which will calm her and give her more confidence in the first place.