Outdoors I like this workout for sprint work. Do the 20 second efforts absolutely all out. This type of outdoor work helps develop both your short time peak power AND technique. While you can try short power work on the trainer, you totally miss the technique work.
ROLLING SPRINT WORKOUT:
1 Set = 2 minutes, 20 second effort with 1:40 recovery
Do 4 sets of 4 with 8 to 10 minute recovery between sets.
Another good workout that simulates a group ride is 2x15 minutes @ sweetspot with a 10 second burst to 130% every minute (then back to sweetspot). Repeatability is more important than peak watts for what you are after. Consider that not being able to match accelerations in a group setting is not necessarily a pure sprinting issue but is often a sign that you just need better overall conditioning so you have more available power when you need it. And, learning to anticipate when accelerations are coming is huge too.