Hi we have sued the Actiheart which uses a branched equation model to estimate energy expenditure. The device is quite expensive but has generates data comparable to indirect calorimetry in variety of intermittent and continuous activities 

Depending on the level of temporal detail you need, it might be worthwhile considering doubly labelled water. For measuring total energy expenditure over a longer period like of time, like days, this may be the best option. However, if you are looking to see how energy expenditure varies at a higher time resolution then a device like the ActiHeart is a validated option which will also give you other data. You can couple other accelerometer devices too like those by ActiGraph or Genea to heart rate monitors and then run an algorithm like ActiHeart uses to estimate energy expenditure. This last option may more flexible to your needs, but is less likely to be validated and may require more time to program etc. 

Hi! The best way is doubly labelled water is the best way to measuring metabolic rates, but I think using ActiHeart provides trustable data. The rate of the device around $1000 

The question is, "why do you want to or need to measure TEE in a group of athletes?"  Although the devices noted above can provide some physiological metrics which would allow you to estimate TEE in a group, it is still an educated guess.  It really depends if you need to know 24 hour values or values for a given short training period. Doubly labeled water can be used for relatively short periods of work (for that technique).  However, the conditions must require higher than normal rates of isotope loss (see attached, we have done this at Ironman).  Compared to specific equations generated using indirect calorimetry - you can see excellent agreement between the 2 approaches.  Other metrics (HR, accelerometry, GPS) might give you more specific data despite them not being able to give you spot on energy expenditure values.