We had a family/friends group cruise a couple of years ago, it was wonderful. We sailed on Celebrity Infinty and just loved it. I would have to say that RCI would be a great choice, as well.
I'd have to agree that you either find a TA to organize the cruise booking and don't attempt to do it yourself. A TA can narrow down the choices of ships/lines for you, then you can present perhaps two choices to your group. More than that and you will confuse some of them and never settle on a choice and let it be known that the majority will rule.
We planned for almost a year in advance, since my husband and I had been on numerous cruises, we were chosen to give advice for other aspects of the cruise itself.
One thing I would suggest is to set up rules everyone can live with for the cruise itself. Our rules were, no rules, no one was expected to try to stay together during the day. We agreed to meet for dinner every evening and then, if a couple decided against dinner, no worries. We did not try to stay together on shore at all and that worked out perfect. Everyone was free to go about whatever they preferred and we had plenty to discuss in the evening.
The only problem we were presented with (and it didn't show up until we were actually on the cruise) was one senior single person. It would have been a little easier on all of us had we agreed to include her in our individual plans, each taking a turn. It wasn't horrible or anything, just something that could have used better planning.
Have a wonderful cruise and have fun planning. Vancouver is an excellent choice to cruise from. While I like Seattle as well, I think the itineraries are just that much better from Vancouver.