Unfortunately I am not at my WoW pc so i can't test this otherwise i would before posting

From what I can gather the commands should be Player, Party1, Party2, Party3 and Party4 (there is no party5)

Taking your Party leaders name as Z and the rest as A B C and D

if you are A:

Player: heals A
Party1: heals Z
Party2: heals B
Party3: heals C
Party4: heals D

if you are B:

Player: heals B
Party1: heals Z
Party2: heals A
Party3: heals C
Party4: heals D

if you are C:

Player: heals C
Party1: heals Z
Party2: heals A
Party3: heals B
Party4: heals D

if you are D:

Player: heals D
Party1: heals Z
Party2: heals A
Party3: heals B
Party4: heals C

if you are Z:

Player: heals Z
Party1: heals A
Party2: heals B
Party3: heals C
Party4: heals D

As you can see.. sadly the only uniform position is Party1 from your clones view. If it's a question of inviting players and you want to be able to heal them I can currently only suggest one course of action.. Name all your characters with a special character first.. æ Ð etcetc.. that way you drop to the bottom of the list alphabetically and any new outsider will take Party2 on all except ur tanks screen.. hope that helps in any way :S or alternatively only group with others that start with an A