I Think My Friends Are Bipolar
Bipolar disorder is definitely a continuum, meaning that everyone has some of the characteristic traits, some of the time. The key aspect of this diagnosis is that bipolar disorder symptoms have to be causing substantial functional impairment. While some of your friends may very well have some symptoms of bipolar disorder, these symptoms may not be causing sufficient distress or impairment for them to seek treatment. If this is the case, your advice, however well-meaning, will most likely not be taken well and may not have the intended effect.
My advice in general would be to refrain from diagnosing your friends and/or family members until they — or others around them — seek your advice about it.
If you are truly concerned that some of your friends may have bipolar disorder, you could refer them to Everyday Health's Bipolar Disorder Center so they can find out more information for themselves. And of course, your experiences would be quite relevant for them if they indeed have a bipolar disorder.