Due to it's name a believe that the seventeenth shard was founded by Hoid and was allied with Hoid before odium started killing shards. Here are my reasons:
A. Hoid was there at the shattering and thus if there had been another shard he would have taken it thus becoming the 17th shard.
B. Hoid seems to have a abnormally good knowledge of the 17th shard and vice versa.
C. Hoid knows the leader of the 17th shard.
D. Hoid has knowledge about the workings of the cosmere thus making him a perfect fit for a job involving shards and the cosmere.
I think Hoid and the 17th shard parted ways because of odium killing other shardholders which the 17th shard saw as a thing irrelevant to the state of the people of the cosmere but Hoid saw it as a direct threat to the people of the cosmere thus causing the two parties to part ways.