You need to quietly take them aside, no one else around! and very very politely ask them if they are aware of the fact that have body odor........and that's all you have to do......
I recently had a guy I work with who obviously didn't shower. I made my boss take care of it.
just leave a note on his/her desk informing them
Buy them a gift set of body spray, soap, shampoo and deodorant and say..Just had you on my mind :D
Some people have hyperhidrosis. Don't be so quick to judge.
Let them come back from lunch and find a gift bag of scented soaps, deoderants etc...just be discrete.
just come right out and say it, don't be cruel, but tell them they have to take better care of themselves they smell bad, and need to shower more often, tell them privately to save them any embarrassment.