The intent is good, and I'm glad you're considering getting moving in that direction.
Now, as to the actual suggestion...
First, forget about experience rating.
I know you've spend a lot of work on it, but the sad truth is it's a complete failure. I hear you've based the rating system on the chess ELO rating, well chess and DG are two totally different beasts. One is a 1v1 game with slow progression, the other is a team game with fast progression (you can become a decent DG player in two week's time, with chess, try years instead).
Bottom line is the experience rating doesn't tell anything about a player's skill : you can have 1100 (good rating) and be a hopeless newb, you can have 100 (abysmal rating) and be a very good player.
Rating players ina game like DG is a very tough problem, and I'm sorry I can only tell you've failed at it without having a suggestion for a better system.
But I do have a suggestion as far as the displayed information : total number of games - win % - win % over the last 20 games.
Not ideal, but a lot more useful.
As for the "on this team %" display...
First, can the connection overlay get data from the game lobby ? Because if you can't tell on which team the player is, the idea is stillborn.
Second, well, that's your topic title : what algorithm shall be used ?
Shall the exact team composition need to be matched or any combination of it's members ? (ie, shall the 2-player premade + 1 random guy be detected in a 3v3 game ?)
What part of the game history shall be considered ? Total games played ? Last 30 days ? Last week ?
I believe that to get a meaningful value for that "team %" would be incredibly difficult.
So my suggestion would be to change the display : on line per player, with additional information for players on the same team : total number of games played on the same team, number of games played on the same team out of the last 20 (if total number of games played > 20).
Mr_Wizard : connected (proxied), 31 games together, 17/20 last
Gank : connected (proxied), 6 games together, 6/6 last
Froboy : connected (proxied), 31 games together, 17/20 last
Gank : connected, 3 games together, 3/20 last
AmaZinG_Hero_ON : waiting for connection
Some redundant information, no information about the team as a whole (only about players' familiarity with one another), but I believe it would be better than trying to sum up everything in one figure.