Is there a way that i can add another user to my account without an additional charge?

Is there a way that i can add another user to my account without an additional charge? if so, is there a way for me to only allow her access to certain form submissions?

If you want to allow a certain user to access your form submissions, then you would need to use the sub-user feature. This feature is however available only on JotForm Enterprise plans.

Else, you can share your submission page: How-to-Share-Form-s-Submissions-Page 

This is a password protected link and in read-only mode. So this is not possible to edit entries when using the submission view link.