We have asked for this over a year and they wont do it. You can't even make ten player groups for PVP. We have also asked for twenty players groups for fleet actions but I don't think it will happen anytime soon. If it happens at all.
I'm guessing it's because either they would have to recode a ton of team buff mechanics, or they're horrified at the prospect of rebalancing team buffs that would affect 10 or 20 ships instead of 5. There would also be no way with the current UI design to deal with 10 or 20 man teams; they'd have to do a whole new "raid" interface with a much more compact UI in order to display them all.
Still, what might give 95% of the benefit with 5% of the effort would be just adding default "Red" and "Blue" chat channels to the PvP instances, and having all players auto join and select the proper channel for their side when the match begins. It might even work to just have a new tab pop up as active in the chat window, with only this channel active and subscribed. And the tab could then go away again after the match.
That would at least let you set your "Target: $target" keybinds and other PvP team chat stuff to the "PvPTeam" channel, or whatever it's universal name would be, so that you could be sure everyone on your "side" would see them. Since the real benefit of teams in PvP is the coordination, this would make it a whole lot easier to at least attempt to make that happen in every match, without a bunch of futzing with invites every time or recoding underlying mechanics.
Having a new tab pop up in chat, automatically subscribed and focused, and seeing stuff from your side happening in there like target calling and coordination, might also help noobs clue in a little better to the teamwork aspect of things. If it was a default and always on feature, it will seem more like the "normal" way things happen in matches.
They recently added this to Champions Online, if I understand correctly. (I have yet to PvP in CO...)
I'd be in favor of this, at least in Arenas where you usually don't get more than 5 players on either side. Especailly if you happen to join later, without teaming, you don't even know where the fight is at. And of course, if you want to have any semblance of teamwork and support, you need those team indicators that tell you who's in trouble.