You can use their name and tag number (battletag) to add them from the battle.net agent iirc. Both of which can be found on the forums for its members. As for the forums having the feature, I heard of no such feature, maybe someone can elaborate on it?
Would like to post this method here. All you have to do is right click the persons name and click “Copy link” or “Copy link address”. Which gets you their user profile link which contains the information needed.
Just replace dash with #: Shadow#126792 and there you go, you got a battle tag.
NOTE: As MissCheetah mentioned, you cannot use this method to add people who don’t share the same region as you.