My 15 yr old has a $700.00 W2 from a summer job, do I add this to my return or does she do one on her own?

You don't include their income on your return. It would only go on their return if they are required to file one.  To file a separate return for your dependent you need to set up a new account.  Online is only good for one return per account.  You can use the same email address for 5 accounts.

How to start a new online return

Https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online 

Be sure on their return they check the box that says they can be claimed on someone else’s return.  If their only income is W2 and under 13,850 they do not have to file a return except to get back any withholding taken out.

Filing requirements for a dependent

https://ttlc.intuit.com/community/children-dependents/help/do-i-need-to-file-my-own-taxes-if-i-m-a-dependent/00/26111

Do not put your child's W-2 on your own return.   Was any tax withheld from her pay----anything in box 2 of that W-2?  If not, she does not need to file a tax return. She would only need to file to seek a refund of an amount in box 2.