Iride01 

Yes you need to do the hill more often than the times you ride with the group. Or at least a similar hill. But not every day.

If you are a masher trying to go up that hill in a high gear, then learn to spin easier gears up the hill. That will leave you with more energy near the top of the hill. You may not be with the rest of the pack, but you'll be able to catch up quicker and will have some energy left to do it with.

Reducing weight of any sort will help you up the hill. My current bike is only five pounds lighter than my previous bike. When I got it, I'd be riding along at a fast speed and wonder where all those hills were that use to tire me on a climb. Then I'd realize I just climbed them.

Single speed bike won't do you any good for riding with a group. Different speeds you travel at, accelerations and climbing will demand a specific gear ratio and cadence unique to you to ride with the group without getting overly drained of energy.

If you get anything, get the sort of bikes the members of your group are riding. Or find another group.