How do I tell a guy I like him without harming our close friendship?

You have 3 options here.

1) Don't give off any signals, don't tell him & remain friends

2) Give off signals and hope he works it out for himself and is then brave enough to talk to you about it

OR

3) When it's just the two of you just say "I don't want you to take this the wrong way, but, I kind of like you, I didn't want to say before because I didn't want to ruin our friendship." See how he responds and if he doesn't like you back, the just move on, if he does like you then Yay! \o/

Good luck