How do I cite a reference in text which has come from another source?
This is called secondary referencing. You should always try to access the original source (Guastello), as Brown et al. may have put their own slant on the information from Guastello.
If you cannot locate the cited source, then you may cite it as a secondary reference. Your in-text citation should refer to both sets of authors, and your reference list should only refer to the source you have read - in this case Brown, Sokal & Friedman.
(Guastello as cited in Brown, Sokal, & Friedman, 2014) OR Guastello (as cited in Brown, Sokal, & Friedman, 2014)
Brown, N. J. L., Sokal, A. D., & Friedman, H. L. (2014). Positive psychology and romantic scientism. The American Psychologist, 69(6), 636-637. doi:10.1037/a003739
Please refer to the official UNDA Referencing Guide (linked the bottom of the page) for more information on referencing in the style appropriate to your discipline.