I love this idea, however I think it should be restricted to party only. Reason being is it would instantly be a huge griefing tactic if you could just do it anybody walking by. This way the emote request would be built into parties and it is already known that this is a willing participant because he is in your party. It saves a step.

To get around the request to participate in a group emote is, that they could make the emotes work like this.
1. Player initiates group emote and the animation. Like /hi5 would stop and wait for 2-3 seconds for somebody to target you and initiate reciprocating the emote.
2. Other player preforms same emote within the 2-3 second time period.
3. both characters line up, sync and complete the emote together.
4. If no other character reciprocates the emote within the 2-3 second time period. The first character finishes with an alternate group emote fail. So you see the character swish his hand with nothing in the air and look sad

Other group emotes I'd like to see: (because I have thought about this myself!)
/hug
/holdhands
/brofist
/groupdance1,2,3
/challenge (this would happen when initiating a duel for PvP. both characters line up scowl and brandish weapon)

The only known example I know of of group emotes is, Entertainer in Star Wars Galaxies. (R.I.P)
In that game you could form a party of entertainers and they would play intruments or dance.
One player would initiate 1 of 5 different songs and play the music track for that instrument. Other players would join in playing the song and play their respective instruments track. There was drums, clarinet, guitar instruments.
Dancers had their own set of dances. You could do /bandflourish as party leader and everybody would play an emote of spinning and the music would play a small interlude variation of the song. it was quite fun.

In game you would listen to entertainers for XP buffs, and extra damage. Depended on song. It was a lot like bard in FF,but you could not use entertainer in combat. Only MMO I know that had non-combat classes that was not crafting oriented.