If the pony reared and charged you, then definitely get another person to help you next time, especially if this is the first time your pony has been in a round pen and tried doing join up. You want the next time you do this to be more certain of a successful outcome. THEN you can try it.
I am not big on join up, but I do do round pen. And for me it's not about making my horse join me and follow me aorund. It's about getting him to honestly listen to what I have to say. So, I try saying a lot of different things, like go, slow down, speed up, look at me, move your butt over, move your shoulder over, back up, follow me, lower your head. If I have my horse doing these things and doing them without too much bad attitude, and he is doing what I ask without resistance and negative push back, then that is enough.
So, for asking the horse to go forward, I mihgt not actdually keep them moving forward the whole time, IF when I asked for a good surge forward, I got an honest try. If the horse blew me off with a la-de-dah "I'll trot this little bit and maybe you'll leave me alone", then I would get real strong and get a real strong response of forward! And then I would just stop pushing and maybe stand or walk slowly and let the horse cruise forward on their own. Afterall, she did what I asked.
Then, ask again with LESS and see if you get an honest forward, but if not, then shazam! The rope snakes out and stings 'em on the behind, and if they go, I let them and take all pressure off. ALL I want is forward, not sustained forward. THAT is step one.