So, I recently read a post stating that Hail of Thorns does not discriminate friend from foe; that EVERYONE within 5' of the target takes damage.  Does that mean that if the caster were within 5' of the target that they would take damage as well?  Is it essentially an Area of Effect spell?  

The next time you hit a creature with a ranged weapon attack before the spell ends, this spell creates a rain of thorns that sprouts from your ranged weapon or ammunition. In addition to the normal effect of the attack, the target of the attack and each creature within 5 feet of it must make a Dexterity saving throw.

Yes - absolutely.

Yes. But it does beg the question of why you’d be making a ranged attack against someone adjacent to you.

Why not?  Hmm.  A better answer is with Crossbow Expert and a loaded hand crossbow, there could be times when it would happen.  Particularly if I could get that extra damage from Hail of Thorns to take down the big bad.  But . . . now I see that I need to be out of the area of effect.  But I do have Fancy Footwork, so I've got the movement to stab, move 5', then fire.  

I was only asking so I would know where NOT to stand.  

Hmm, would you have enough object interactions to stab, put that weapon away, draw your ranged weapon and then fire? Depending on how deep you went into rogue, if you have Evasion you might also just be able to take no damage if you roll high enough. 

One of the features of Crossbow Expert is that if I have a hand crossbow in one hand and take the attack action I can fire the crossbow (at 5' without disadvantage).  If in round one I cast Hail of Thorns and then grabbed my crossbow (my rapier would already be in hand) then in round two I could advance, stab, fire, and move away.  In that Hail of Thorns is a Bonus Action spell, I could actually stab, cast the spell, Move, then in the next round fire.  

These are very situational and it may not ever be advantageous, but still, I ask the question so that I don't end up hurting myself just because I didn't know any better.

I could see myself going after the Big Bad, stabbing twice (two attacks/round), getting my Dreadful Strike, firing a crossbow with Hail of Thorns, then throwing in my Sneak Attack just to be mean.  And then hearing the DM say, "Okay, make a Dex check..." That would be sad.

Now I know to move before firing.  

Hail of thorns is a bonus action to cast, so you couldn't cast it AND fire the hand crossbow as a bonus action on the same turn. But if you cast it on a previous turn and haven't had it activate yet/haven't failed a concentration save from damage, that could work. 

Also - just be aware because it's likely to be relevant if you're using a sword and a hand-crossbow at the same time - don't fall afoul of the Ammunition property. It requires you to have a free hand in order to reload the hand-crossbow. The Crossbow Expert feat lets you ignore the Loading property - but not the Ammunition property.

Yes . . . I assumed that was implied.