Ok so we know that the Seventeenth Shard is a society that exists in the cosmere, however we don't know how they are organized or what their intentions are. I thought it might be a good idea to compile a list of who we think might be in it and what they are up to. I'll start with some of the ideas that I had. Note that this is mainly speculation and not a lot of evidence is backing this up.
1-Hoid, I think this might have been confirmed but I'm not sure, If there is a secret orginization of planet hoppers then he's practically guaranteed to be involved somehow.
2-The Selians in the Interlude
3-Sigzil (sort of, I think Hoid might be recruiting him but he's not aware of the society)
4-Rock's Teft's Parents, Thanks for the correction CrazyRioter Well I think that that group might be the Seventeenth Shard's headquarters on Roshar or they have something to do with them.
5-Dalinar Kholin, not a member yet but likely being recruited.
6-Sazed, If the SS are trying to restore Adonalsium then I think that Saze would want to help and they would be foolish to not let him join. He probably at least knows of their existance.