I have a lvl 13 wizard and I can’t find the spell crown of stars in the lists for 7th lvl. I have xanathars Guide to everything shared with me. 

Is the character in the campaign that has content sharing enabled?

I guess you've already prepared another spell at LvL 13. Or maybe the ""shared book"" dosen't allows including spells on your character. Or maybe your account is not in a Tier that allows sharing content.

Quote from Dertinus >>

I guess you've already prepared another spell at LvL 13. Or maybe the ""shared book"" dosen't allows including spells on your character. Or maybe your account is not in a Tier that allows sharing content.

 
So many things wrong with this. 👀

1. Your spell preparations has no effect on whether you can see the spell or not. 

2. Content sharing allows adding of spells - it's not possible to prevent this.

3. There's only 1 tier that allows content sharing and only person in the campaign needs have it - whether you, yourself, have it is irrelevant if others have it. The OP confirms the book is being shared with them.