A little help with proper possessives

[jakewriter82]

Story has proper name used like this:

After sitting a while, the Johnsons friend decides to leave.

Is it Johnsons' or Johnson's?

I'm inclined to think it's Johnsons' but I'm not 100 percent sure.

Thanks in advance!

 

[ColbertNation]

Johnsons is plural; possessive of that is Johnsons'

 

[DemoChristian]

That's right, though it's a weird-looking sentence.
Why not "the family friend?"