Just being friendly or something more?

Guy at my work who I've only spoken to a handful of times (and very briefly on all occasions) came up to me alone and invited me to his birthday drinks.

He then went on to say I should add him on Facebook. Later that night I added him on Facebook then he sent me a message with his mobile number and asked me if I was coming out or not. As it happens, I couldn't go.

The key issue in my mind is that he has a girlfriend. Maybe I'm overthinking it but I barely know him and he asked me to come to his birthday and gave me his phone number. If I had a boyfriend and he did this with a girl he just met i'd be a little suspicious, but maybe that's just me.

What do you think? Was he trying it on or just being friendly? I appreciate there is no definitive answer here but just want to see if people think I'm right to be thinking this way