I just cold approached two women on my own last weekend and a few weeks before that I cold approached a group of four on my own. Both were at patio bars. Both times, the girls were standing up.
In my experience, girls welcome a guy to join in their converstation but you have to be totally calm and confident. Have a question you want to ask the group, or a few people, say "excuse me, I don't mean to interrupt but I had to know..." and ask your question. If you get any attitude, just laugh it off, remain un-phased, and play along with it. My experience is that when I walk up to a group solo, the girls open their eyes wide in astonishment that a guy on his own who isn't drunk, slurring his words, putting on a front, or seeming at all nervous, would ever approach a group of girls. The key is to remain calm and confident and have the mindset that you're just there to have an interaction and aren't trying to impress them and aren't worried about their impression of you. For example, the last pair of girls I talked to, when I gave them my name, one of them said, "That's my cat's name." I laughed and said, "Who the hell names their cat that??" Girls are simply NOT expecting this from a guy on his own in front of multiple girls. They love it.
If they were seated, I'd do my approach standing and if the first few back and forths went well, I would motion for a girl on the end of the table to scoot over so I could sit down, as I moved to sit down. I'd that wasn't an option due to individual chairs or not enough room, I'd just stay standing. No big deal.
Bottom line: Just do it.