Hey, welcome to The Goat Spot! Glad you're here! Congrats on the new addition to your herd!
As long as she doesn't get bullied, she can eat with the other girls. I would definitely feed her alfalfa hay or grass hay with alfalfa pellets - they need the calcium from alfalfa hay when pregnant, lactating and/or growing.
Make sure she has access to clean fresh water and a good free choice, loose mineral - those are every important for goats. I really like this brand: https://www.tractorsupply.com/tsc/product/purina-wind-rain-storm-all-season-75-complete
When she is a month out from kidding, give her CD/T and a dose of selenium gel. If she has not received any CD/T before, she'll need some about two months from kidding and another dose when she is a month out from kidding. Same with the selenium gel, if she is not getting any minerals and/or is showing signs of being selenium deficient (weak pasterns/ankles, bend in tail) she will need the gel sooner - same goes for giving copper boluses, copper deficiency signs include rough/dull/faded coat and loosing hair on the end of tail.
She most likely won't need grain until her last month of pregnancy, but it really depends on her condition and how many kids she has. You don't want to feed lots of grain early on because there is a risk of making the kids big and harder for her to deliver. You also don't want her to be skinny. Find the happy medium - look up scoring a goats' body condition. It would be a good idea to ask what her current owner is feeding her.
I would ask her owner if she has had a fecal done recently to tell if she has a worm load. If not, it would be a good idea to do that and check her FAMACHA score as well.
There are lots of things you should have on hand for when she delivers, some things necessary while other things for "just in case" - it's always a good idea to be over prepared than under. I would recommend doing lots of research on goat birthing and kidding kit supplies. Watching kidding videos can be very helpful too. Not trying or hoping to scare you, most goat births go very well requiring no extra help from us while others can be a nightmare if one is not properly prepared.
I would love to see a picture of the new doe you'll be getting!
Hope this helps!