Simple answer? To ride faster...ride faster. Meaning, for a well-conditioned rider with good base-miles, just add some speed work (intervals, hill repeats) into the mix.

However...adding speed work without a good base usually ends badly (injured cyclist). Not sure if you gave enough information here, but if you are only riding one or two days per week, chances are good that you don't have enough base miles for the year to begin speed work. Have you considered other ways to get out on the bike a few more days per week (commuting? running errands?)?

Once you are riding 4-5 days per week, you can pick one day per week to work on your speed/skills. Where are you getting dropped by your riding group? Hills? Do hill repeats. Flats? Intervals. If you are getting dropped on descents or turns perhaps you need to fine-tune your cornering/descending skills. Also make sure that you are comfortable drafting and that you know how to keep yourself out of the wind. Riding "smart" can go a long way if you aren't particularly fast