I guess one thing to remember here is this. These are relatively new people to you. So when you text them, even if you don't get to a point in a conversation where you can ask you out, if they stop texting you back. STOP. Also if they give you a definitive conservation ender, just stop and give them the last word. Nothing wrong with that. That way you're making sure to only give the attention you're getting the first time you text them. Or call them. Don't leave a message if you call them, or if you do just be like "Yo, what's up, this is Nave, call me back". Because that's all you want, is a chance to talk to them. If they pick up, obviously you're good to go.
Let them talk more. I think I remember your age range correctly, so yeah, most of them to need to fill in much dead air for them. "Uh-huh", "wow that's cool", "yup" and "it sounds like everyone you work with is so mean" are perfectly acceptible answers. Okay, maybe I'm just being silly at this point.
Yeah though, don't text them ad nausum. A good simple ask out is, just text: "Hey, this is Nave, what're you up to?". See this is an non intrusive question because they kind of already know what you're getting at, but it gives you information up front as to whether it's an appropriate time to ask them out or to come hang. You might also get some insight into what they like to do, so it's not a bad way to start a conversation. Lots of places to go from there.
I dislike "how're you doing" because unless the answer is really bad, in which case you didn't want to know anyways, the answer is gonna be the stock "good". I know this because it's the same stupid stock answer I give. How're you? Good. As opposed to: "oh, I'm chronically worried about money, and trying desperately to make as much as I can, so I can maybe retire one day. My cat's been sh***ing OUTSIDE the litter box! And they cancelled Avengers: EMH!!!!




". Nope, that's a normal day, it becomes "good".
Also, if you can think of a joke, or a good opener: mull on it a day. Sometimes I'll write and rewrite the first text I want to send someone, and honestly I don't feel there is shame in that. Just don't keep a record of that kind of thing. I think it's perfectly fine to rehearse your own stuff. I always think about my audience, the person I'm about to talk to, what we've talked about, and then I'll come up with something kind of smooth off of that. It's got to be like the "un-line" though. I always think like that.
To me though, getting them to approach you is great.