Companies specializing in trekking can organise both individual and group treks. One major advantage to dealing with someone close to home is that it's easy to communicate by phone and the agent can assist you with travel to and from Nepal.
On an arranged trek the group must stay generally on its prearranged route and, within limits, must meet a specific schedule. This means that you may have to forego an appealing side trip or festival and, if you are sick, you will probably have to keep moving with the rest of the group. You also may not agree with a leader's decisions if the schedule must be adjusted because of weather, health, political or logistical considerations.
You will be trekking with people you have not met before. Although some strong friendships may develop, there may also be some in the party you would much rather not have met. For some people, this prospect alone rules out their participation in a group trek.