It's great that you are reporting on your progress! Don't worry if you failed this time to make contact with them, if it was easy there wouldn't be an issue in the first place, isn't it? Since you are seeing them today, and you think they noticed your efforts to talk to them, I think it's a great idea to play with that
>"guise, i tried to call you out yesterday, but .. im too shy.. so .. i didnt.. what yer doing next friday? wanna go out?"
Sounds good, but why not "Hi guys, yesterday I tried hard to talk to you, but in the end I was too shy...smile So... I'm Oneitis, what's your name?", and proceed from there talking about their activity at the gym, and whatever the conversation goes into. And if you manage to arrange an outing, even better, but it's not required.
I think the trick is to get started. Just do like when you throw yourself in the water from high places. Stop thinking, throw yourself, and then you have no choice but to go on. In this case, stop thinking, say "Hi, guys", and you are set, no coming back. Stop thinking/say "Hi guys"/you're off.
Let us know how it went!