Who should I invite to my wedding?

I got my current job about six months ago and was already engaged. Our wedding is set for August. A co-worker got engaged in October and is getting married in April. I'm pretty sure the invitations have gone out, and only some people were invited. I was not.

I'm not particularly close to the girl who is getting married, but I thought it was polite to invite all or none of your co-workers. I was planning to invite everyone in the department to my wedding, even those I don't know very well (about 16 people, with spouses). I understand that budget can be an issue, and I'm not particularly hurt not to have been invited to the other wedding.

Does this mean I don't have to invite everyone at work to my wedding? How do I invite just some people and not others? I saw an invitation on someone's desk, so I don't want to hand them out at work. I also don't want anyone to think it's just a gift-grab; I'd truly like to celebrate my wedding with the people I spend time with every day, and show off my brother's cooking (he's catering the reception). What are my options?
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