I will be traveling with a group of 10 arriving into SJU this summer. All the charter companies I have found need to use 2 planes and all the cost that comes with two planes. Anyone know of a charter company that can seat 10 passengers + pilot?

I don't think you will find a aircraft that meets what you need without jumping into something very expensive with 18 or more seats. Even if you found a 10 passenger airframe it's not likely it would also carry all the bags. 2 smaller airframes in the 6 seat range is probably your best bet.
G

Why would you not just get 10 tickets on Seaborne and save some money?

Times just dont match up well. If I had 8 passengers and could use one plane - it would actually be cheaper than seaborne - its that extra 2 people that throw me over the top and need two planes. Maybe I will just un-invite 2 people.....

Draw straws and send two people via Seaborne/cape air/ air sunshine etc.

And average the total price over 10

Have you looked at the commuter book from cape air? You could get two of them giving you round trip for 10 people. Cost might be similar or better than charter?

Yep. And I'd probably consider cutting those poor souls a small break on the total for getting stuck on commercial.