There are many times when you will find a group that will stay together for many hours. You may also be into a great dungeon, and suddenly hear from the healer, "Got to go, baby-agro."
The good news is that there is a ton of solo content and quests if you cannot or don't want to find a group.
When you take on solo mobs, you will get low-to-medium exp. Solo mobs also drop less valuable loot. All that is for good reason - why get great loot from an easy kill?
When you take on heroic group mobs, you will see a significant rise in exp and loot. In fact, I was in a gnoll dungeon (Blackburrow) with a group and gained 2 levels in one night. We had members come and go, and at one point I had to take a dinner break.
Of course heroic group mobs are much harder, as we died several times in that dungeon.
You will also have several chat channels available to talk to others, find a group, or announce you have something for sale/trade. There is always someone on the other side.