The current conventional wisdom says high intensity interval training (HIIT) or polarized training to improve sprints and short bursts such as closing gaps or tackling many short, steep hills, typical of the roller coaster terrain we encounter across much of the US in states that lack many real mountains. (Check Dylan Johnson's channel on YouTube. His videos are all summaries of evidence based research publications, not merely his personal opinions.)
Most training programs for HIIT and polarized training emphasize avoiding tempo, sweet spot, or repeated threshold efforts. That pretty much rules out group rides for these training sessions. Unless it's a very disciplined group with specific goals, most group rides quickly become tempo or harder rides, with the pace set by the group leader.
Check your terrain, especially the segments that give you the most trouble. Gear your maximum efforts to that length of time, along with easy pedaling to recover. If you're on an indoor trainer, set up a timer to suit those routes. Cyclemeter and a couple other apps offer programmable timers.