Minecraft – In Minecraft, I want to testfor if a player is on one team or the other

What you are looking for looks something like this:

{Inventory:[{id:minecraft:written_book,tag:{pages:[0:"\"Test\""]}}]}

But the teleport command does not support NBT tags. However, there is a workaround using scoreboards.

First add a scoreboard objective:

/scoreboard objectives add HoldingBook dummy HoldingBook

Then run these 2 commands on a fast clock:

/scoreboard players set @a HoldingBook 2 {Inventory:[{id:minecraft:written_book,tag:{pages:[0:"\"Test\""]}}]}
/scoreboard players remove @a HoldingBook 1

Now, you can teleport the players holing a written book with the word "Test" on the first page, and for example on team blue;

/tp @a[score_holdingbook_min=1,team=blue] <your-coords>

Hope this helps.

To "lock" the inventory, you could summon a mule with a saddle: /summon EntityHorse ~ ~ ~ {Type:2,Tame:0,SaddleItem:{id:329},ChestedHorse:1}. Summoning a horse that is not tame and giving it a saddle will make it rideable, with an inaccessable inventory. Also, testing for a mule will work using the /testfor @e[type=EntityHorse] {Type:2} syntax, so you may simply have an issue with your setup. I hope this helps!