If they seem friendly and start conversations with you, yet you need advice how to ask for their number to be their friend I think you may need to work on your self confidence. I totally don’t mean that to be insulting – just giving honest advice.
Despite the OP having the name reb I’m assuming they are a girl. If you are a guy why are you looking to be “friends” with frum girls? If you are interested to date them either tell them so and ask them if you can take them out on date or if you want to use an intermediary give them the person’s number and say you would like to date them but want to go through somebody and if they are interested they can call that person.
If you aren’t looking for a date then again, why would you just want to hang out with frum girls? That could lead to a range of innapropriate situations.
If you are indeed a girl and looking for new friends just be straight up and say, “We should hang out/get together sometime – what’s your number?” You’ll find that often in life being up front and honest is the best way to communicate with people. Nothing to be embarrassed about by wanting to be their friend – it’s a compliment to them.