This would be a helpful addon but nobody has done it.

However there is a way if you have not created your team yet:

Naming

Believe it or not, the names you pick for your characters make a difference. By picking logical names you will be quicker to react and it will be easier to tell your characters apart. I personally choose Azin, Bzin, Czin and Dzin for my mages names because they are easy to tell which is which. Xzin is my priest - nobody is going to confuse X with the first few letters. Some people choose A,B,D,E as their secondary characters and C as their main, or A,B,C,D as their secondaries and E as their main. The choice is up to you, as long as you are able to differentiate which is which then go for it. I don't personally see why E,F,G,H and Z could not work - but it just seems to make sense to make this as easy as possible. You will have enough going on as it is and fast and accurate recognition of who needs healing when is critical. Also, by picking X as a main, you reduce the chances you will randomly target somewhere else when / if you ever /follow x. Not very many characters names start with X.

The TRULY devious will pick names that sound nearly the same over Vent/TS to better obfuscate target "training" and subsequent burst damage as well as generally foul up target selection against you in BGs and arenas. This can take the shape of either names that are very similar in pronunciation like wilboor wilbour etc or nonsensical names like lillilili and liililil etc.

Also keep in mind there is now a name change service.

Ziran pointed out that the order your characters will appear in a group is determined by when you create the characters.

WoW orders the party in the reverse order of the absolute character creation time. In other words, you want to make your "main" last, and your others in reverse order.

If you want:

Xzin
Azin
Bzin
Czin
Dzin

you need to make them in this order:

Dzin
Czin
Bzin
Azin
Xzin

Also of note: you can reserve names by creating level 1s with whatever name you want. Upon deletion, that name is freed up for (presumably) you to take. This is helpful to ensure all 5 names are available.