Playing in a team with one of my listed friends; however, I cannot warp to him. The button is grayed out, and I get text saying I cannot warp to him because he is not my "buddy." Can anyone explain this for me?

You get to use that when you refer someone only, methinks..
I have one guy I tricked into playing STO under the referral program and I can use that "warp to" feature with him, not that he plays anymore, can't use the warp to feature on anyone else on my friend's list.
I'm of the opinion that's the only time you can use it, when you "refer" a friend as opposed to just being one.

Make it cost credits in the same way it costs credits to transwarp to a mission location.

Pretending to be your friend but is secretly plotting to destroy you. Your helm sees through this plot.