Bresail, it sounds like you have two separate amplification issues to solve.
1) Being loud enough to be heard over 15+ other drummers
2) Being mobile
(if there are other issues, please feel free to let me know)
the main problem is the mobility. There are "micro amps" which are small enough to fit in your shirt pocket (assuming you have a shirt pocket, I'm a t-shirt and sweatshirt guy myself)
http://reviews.harmony-central.com/revi ... o+Amp/25/4
Then there are mini amps or busking amps
These fit the exact description that you are looking for as far as application goes. You can mount them on a drum or on your person and go at it (the mini amps are probably a bit too heavy to pull this off on a regular basis, considering the weight of the djembe that you have to start with)
The problem that you are going to have will be with the volume and sound quality. Generally speaking a djembe puts out a lot of sound, and in order to reproduce that sound, you need a big speaker (and the power/batteries to drive that speaker) Smaller speakers will more than likely distort at the volume necessary for what you describe. They would probably sound "ok" at lower volumes, but at that point they will be quieter than your djembe alone.
Another thing that you would probably run into with this kind of setup is finding cords short enough not to trip you up between the mic and the amp, you probably need a 2 foot cable at the longest, the smallest I usually see is 4 feet. Shorter cables tend to be "patch" cables intended to go between foot peddles and would not be appropriate for microphones.
The recommendation that Michi is making is the one that makes the most sense to me. Wireless microphone (so you can be mobile) connected to an amp or PA.
If you are doing drum circle facilitation from a djembe, I would recommend using a PA with multiple speakers. That way you can have the PA at a lower volume spread around the group, instead of blasting from one side so that the other side can hear you.
If you are facilitating a drum circle, my advice would be to not use a djembe at all, but to use a cow bell instead. Less intimidating for the participants, loud loud loud! and easy to put down if you need to help someone with what they are doing and pick up again to jump back in. But this is getting into another whole conversation...