In reply to SeanDT:
Whilst exiled_northerner is over-doing it a little, what I do find a bit frustrating about posts like these is that they are so open-ended in their nature that it is clear that the enquirer has put very little effort into actually researching the challenge themselves, and is just seeking answers on a plate from the forum community.
I remember many years ago I researched, planned, and led a group of 8 over the Welsh 3000s (including leading training/reccy weekends) - this was before such times that online forums had become ubiquitous. It was doing the 'leg-work' myself which gave me the confidence that I could lead lead the group, whatever the situation (in the event, we had thick mist on the Carneddau, and crossed Crib Goch in the dark, but completed successfully, achieving what, for all of us, was one of the best days of our lives).
When you ask questions like 'what route should we take up the mountain' or 'should we hire a guide' you give the appearance, rightly or wrongly, to have done no research, and have little idea of what you're doing, thus inviting comments from the likes of exiled_northerner and myself!
For what it's worth:
No, don't hire a guide, in summer that's massive overkill.
Going up and down the PyG track from Pen y Pass should take you around 3 hours (from personal experience taking an average group up that route on a 3 Peaks Challenge).
From Pen y Pass to Barmouth it's 32 miles, mostly downhill or flat. If you are all fit, and on road bikes, then 2 hours should be plenty to do this.
Barmouth to the Craig y Castell car park is 8 miles. Should be able to run that in an hour. Maybe 1h15m for contingency/faffing.
Then up the Pony Path to Penygadair. Been years since I've done it, but it looks like it's around 8km return, and climbing 700m, so that's under 3 hours up and down according to Naismith.
Allow 30 minutes each for food at Pen y Pass and Craig y Castell.
That gives you a total time of 10h15m, so in summer you'll have plenty of daylight to play with.
Hope that's helpful. but you really should be able to work that out yourself imho.