Let's start with pronouns.
Which is correct?
- Give the report to she and her boss.
- Give the report to her and her boss.
If you answered "she and her boss," you're making a mistake that's more and more common. No matter how common, however, it's wrong. It's a big, honking mistake, and you run the risk of sounding, how shall I put it, less capable than you probably are.
Without getting into the grammar of the situation, which involves the use of objective and subjective pronouns--and that can be a yawn--let's just look at the way to solve the problem.
Would you say, "Give the papers to she"? Probably not. Then don't say "she and her boss." If you wouldn't say it in the singular, don't say it in the plural.
Let's try another.
- Her and I are going skiing.
- She and I are going skiing.
Same test. Would you say "Her is going skiing"? No. So don't say it just because the subject is plural. I repeat, if you wouldn't say it in the singular, don't say it in the plural.
Simple. Here's a little rhyme to help you remember:
"When plural pronouns give you doubt.
Take one away; then try it out."
You'll speak and write more confidently--today.