As you may have heard, the Celtics won't be fielding a summer league team this year, and instead will be holding its own minicamp. Marc Spears has details on some of the invitees :
The Celtics will hold a minicamp for 16-20 players at their Waltham training facility from Wednesday through Saturday. Celtics forward Glen Davis, guard Gabe Pruitt, and first-round draft pick J.R. Giddens are expected to participate, as are ex-NBA players center Jackie Butler, forward Lawrence Roberts, and guards David Noel, Omar Cook, and Mustapha Shakur. Forward Bill Walker, a second-round selection by Washington who was traded to Boston, will not participate. Ainge said Walker will likely undergo minor knee surgery this week.
Undrafted free agent Brian Butch, a center from Wisconsin, will also be attending .
There aren't a lot of big names there, but keep in mind that this is basically the equivalent of our summer league team. The Celtics are in all likelihood just using this camp as a means of finding a guy who could be the 15th man on the roster, with the potential to do more if everything goes perfectly.
Also, I'd expect the quality of that list to increase a bit after July 1, the first date the Celtics can officially talk with free agents. I'm sure extending an offer to camp prior to that deadline could be considered tampering, which is why the only names you're seeing reported are guys who are currently out of the league.