I have a 5 month old a friend has a 5 yr old and its her birthday .. is it a good idea to take a 5 month around that many people and it flu season?

Not really: Young children, especially under 6 months, are more prone to serious infection from the flu and should avoid unnecessary contact with sick people. A room full of 5 year olds is bound to have at least a few viral illnesses spreading around! you can take your child out in other situations where you can keep people from touching her, but a birthday party is definitely higher risk.