I have been invited to peer review a manuscript for a reputable journal. This is my first so I have a few questions to the more experienced Are PhD students generally invited for such reviews? I was under the impression they would prefer someone with a Doctorate degree or more experience in the field.
What are the advantages/disadvantages of accepting to review? 
The general discipline that the paper treats is related to my field of study but my active research is not necessarily related. Is it still OK to accept the invitation to review?
It is not unusual that PhD students get invited as reviewers, after all a PhD student will likely be a true expert in the field of the PhD. If you are a student during your first years of study, the request may be a little premature, generally speaking but if your are in your final year then it will be a good experience. What you need to consider is whether you can provide an insightful review of parts of or preferably the entire paper. You should have a sense of why you were invited, i.e. why your expertise may have been asked for.

Peer review is a vital part of the publishing process so getting experience of reviewing other's material is very worthwhile. If you continue in academia this will be expected of you so you will have to start sometime. You can definitely add reviewing for journals in your CV, not mentioning what you reviewed but certainly for what journal you have reviewed.

If yo have not done a review before you should probably ask someone (or preferable more than one) more experienced within your field for a brief outline of what should be included in the review and how to formulate the review.