What to Wear: Wedding Guest Edition

One of my friends is getting married next month and I just realized that I'm not sure what I'll wear. I know, I know, first world problems.

Anyway, help me narrow the field?

I'm really liking the necklace in the bottom corner and it's made even better when you factor in a 50% discount (yeah, summer job).

Apparently, I am a fan of tank dresses.

I don't own a lot of yellow and am really debating the yellow shift dress (bottom left).

Please throw out some opinions or alternate suggestions. Where are your go-to shops for dresses?

Sources:
Top yellow
Bottom yellow
Top red
Bottom orange
Navy tank dress
Green dress
Blue dress (with v-neck front)
Necklace