I'm wondering if your granddaughter experienced this challenge last year in kindergarten? If not, then you can honestly reassure her that she has what it takes to be a good friend. You can also let her know that sometimes it takes a while to get to know people and to let them get to know us. You can also talk to her about what she thinks is important in a real friend (the only kind worth having). Help her see that what matters most is honesty, respect, caring, etc. Do the kids in her class have those traits? If she doesn't know, then you might challenge her to find out. On e more thing, if your granddaughter had challenges making friends last year and this seems to be more of the same, I suggest her mom have a conversation with the teacher. There are plenty of things a teacher can do to encourage friendship-making skills at the first grade level. Mom can be encouraging in that direction as well.
My son had trouble making friends in first grade as well. I volunteered in the classroom one day to get a lay of the land and figure out who I thought might be a good fit. Then I tried to set up play dates with a few kids who seemed like they might have things in common with him. While I can't say that every one was a success (one was a HUGE failure), he did end up becoming pretty good friends with one of the boys. It helped him be more positive in school. I also talked to the teacher, to get her to help as well. She did a good job of helping him move into groups that were working on projects he might like. It's very early in the year to know if kids have something in common-- maybe your granddaughter just needs a little help finding something in common with others in her class!