I've never used one but it seems interesting. How does it work exactly?
First of all, you have 100% chance of hitting your opponent with Thunder. Getting advantage of this and the Sp. Attack/Speed base stats of Starmie which are high enough (100 / 115), you can have a guaranteed thunder for your opponent. Also you can teach Starmie Dazzling Gleam (In Doubles try to have moves that can hit more than one opponent in one turn; ex: Heat Wave, Surf, Rock Slide etc).
Well, I think there's not much to do with Starmie in a rain dance team. Some people even say that they don't like the idea of that type of teams cuz they end up as monotype teams which means most of the pokemon in such teams usually share the same weaknesses. Well, you can find ways to eliminate somehow those weaknesses; For example in a rain dance team you can get rid of Electric Types with the help of Seaking. Seaking has the ability Lightening Rod which absorbs all the electric type moves. Also try to involve in the team other pokemon rather only water types, and make sure they can benefit from Rain dance.
You can match up pokemon, not only in a rain dance team but in doubles, in general, by thinking how the one can help each other and at the same time, trying to organize the team like a normal one in singles (Leader, Attack, Defensive, Sp. Attack, Sp. Defensive, Supportive). If you check again the team I wrote, you will find out that it is wrong though. if you see it from that perspective you will realize that a defensive wall is missing and there are so many Sp. Attackers.
[ Leader: (Starmie) Attack: (Swampert - mega) Defensive: ( - ) Sp. Attack: (Heliolisk, Ludicolo, Vaporion) Sp. Defensive: (Jellicent) Supportive: (Politoad) ]
Politoad's Drizzle saves you a turn, because you have rain as soon as Politoad is in battle. Swamperts' (mega) / Ludicolo's Swift swim makes them really fast. Also Swampert is immune to Electric types which this may be helpful for the team. Heliolisk's Dry Skin makes it recover a little bit of its HP each turn when it's raining. Vaporeon's or Jellicent's Water Absorb is a good way to heal their HP as you can hit them with a water type move (ex. Surf so you can inflict damage to your opponents at the same time). Having in the team a pokemon with swift swim, you can use a water type move on them first before the faint and keep them in battle.
There are many pokemon that can fit in a rain dance team and I'm sure, much more interesting combinations can be made. I hope this was helpful~
I was just making a joke about double team the attack, not an actual doubles team.
But here's some real advice, look at Pokemon global link, they have a list of the most used Pokemon in doubles, and the moves they use. There's also pokemon showdown that you can practice teams and watch battles on.
I agree with you
@Rocket Grunt Trever but why not trying to make his own team rather than just copying it from someone else? I think, building your own team in showdown can be way more fun!