There's no right or wrong here just because she's at work - she's a girl, you guys get along, see where it goes. That's all that matters in life.
Even if you met through a dating website, it doesn't change anything.
It's always "let's see what happens", you could be friends, you could be fuck buddies, you could end up dating in the future, or nothing could happen at all. Don't be outcome dependant, just enjoy her company.
Inviting her out to the party is easy - you can say something as simple as "Hey we're having a party (give details) you should come!" it's just the way you do it. And that takes practice - to just relax and do something because you think it'll be fun, not because you're concerned about the outcome. You should have half a mind to weave it into the conversation but don't be too forceful about it, but don't leave it too late either - your delivery will need to be appropriate depending on the situation (and that's what makes it natural) eg. if I was thinking it hadn't come up in convo yet and it was getting late, when I'd finished talking to her one time, I'd say it in a way that it just occoured to me to invite her because I was talking to her right then and she seems cool and it might be fun. I mean, it's relaly dependant on the situation though, you have to be creative and flexible.
I'm about to do this this week actually

It's a drawing based event and if drawing doesn't come up in the conversation I'm just going to open her with "hey have you done any of your drawwing recently" and then "we're having a drawing night" etc. and tell her how it works. I've been watcvhing the signs for a while, she's clearly expressd interest in seeing me naked and that she likes modelling herself. So I'm using that knowledge as a kind of a hook.
But while it's good to slip it into conversation, or have a hook, you shouldn't depend on that. There doesn't HAVE to be a reason, and you'd seem wierd trying too hard to come up with one. You just say as above, it's just a "you should come along because it'll be fun" thing. and the key is it's all in the confident and positive way you do it.