I thought it would be fun to mix them up so they could get to know different people and they could always move around after dinner.
Some of the most fun weddings I have been to was where I have been seated with strangers.
The last wedding I went to had me seated at a table with people I knew very well. We had a great time together though we did not mingle very much with the guests we didn't know.
I went to an out-of-town wedding once where I didn't know anybody (my husband was performing and I was his guest). We sat at a table with a bunch of strangers who quickly became friends. Two of them even came to our wedding.
I think it depends on your guests. Family would probably prefer to sit with each other, while friends and other guests may be more open to mixing it up. Perhaps you'd consider mixing the bride and groom's family members at some tables, and mixing the friends at other tables.
A few things to consider... Is the person coming as a single or couple? I went to a wedding alone once and knew nobody! It was a little awkward for me because I tend to be shy in strange situations, but my table mates were so nice and friendly and I appreciated my friend seating me with them. So if someone is coming alone and shy, careful where you seat them. At our wedding we had a few guests who spoke no English so we sat them at a table with people who spoke their language. We had a couple guests who didn't get along so we sat them separately. As for couples, we tended to keep groups who knew each other together, but mixing is fine too.
