Best way to put weight on two young horses?

Hi, a guy is keeping some horses on my farm in return for building some fences for me. 2 of the horses are way too thin, and grass is not enough for them - they are only 2 years old, and they are small, really ponies, I suppose. I figure I'll just have to supplement the grass since the guy who owns them doesn't have any money at all. Maybe I should just buy them from him?

Should I give them beet pulp? I give it to my senior horse in the winter, seems to keep him healthy (along with his equine senior). Or can I get by with just some sweet feed? His other horses appear to be just fine, easy keepers, but these two, I'm concerned about.

They are in a huge pasture that is about 40 acres right now (6 horses total). But the grass is not as good there as in the little 4 acres pasture that my 5 Angora goats and my own two horses are in. I'm thinking about moving the two little horses into that pasture - it has a thick growth of brome in it. Or, should I be giving them hay? I've never run into a problem with horses not gaining weight before, hopefully some of you might have some good advice.