In my emails with a university professor, speaking of new students, it says:
looks like A, B and C will come. Should be a party!
What does "should be a party" mean in the context?
It means "I expect it will be enjoyable because there will be lots of people"
A "party" is a "a social gathering of invited guests, typically involving eating, drinking, and entertainment." (Oxford) For example a birthday party.
In a casual way the professor writes that three other people will come, and so he/she expects that the gathering will be fun, because many people are coming.
It is casual and light-hearted, and not completely serious.