Not at all. They have different meanings. 'Join in' means 'participate in'. You see people playing a game, for example, and they'd ask you to join in. 'Join up' means enlist, usually in the army or a similar organisation. Neither expression fits with 'course' : 'sign up for a course' is probably the word you want here.

They mean two different things. "Join in" means to "start participating" so you "join in" a conversation. "Join up" means to enroll or meet, so when you go into the army, you "join up". You also use it in sentences like, "I'll join up with you when I am finished." In your phrase as written, you need simply "join". "A course I'd like to join."

To join "up" means to sign up for a specific group, usually formally. You "join up" if you're enlisting in the Army. To join "in" is more informal. You "join in" with a group of friends sitting around a campfire or doing some work in the yard.