Going to meet Bio Dad, but he not his family?

Don't feel snubbed, it is just awkward for him too, and perhaps in time he will change his mind about you meeting others. Sure, you can ask to meet others if you would like too, there should be no harm in asking. As far as conversation topics, even though there may be some uncomfortable feelings, questions about jobs, hobbies and other activities are good ways to find out about another. Just think about what you and your friends talk about, and take it from there. As far as clothes, what you said seems fine, there is no need for formal clothes, just wear decent clothes that are comfortable. Best of luck, there is likely to be some nervousness on both sides, just admit that, and be open, honest and friendly. Hope is all goes well.