I tend to allow the players to make whatever characters they want. I then try to find ties (with the players help) within the group to make them somehow know each other or make their backgrounds intertwined. I must admit the system I play makes this fairly easy (edge of the empire).
Once the game starts the best thing you can do to tie everyone together is to put them in a situation where they have to start out working together. (Aka in medias res). Start the game in progress don't let them not join the party because the initial meeting didn't go so well. Usually this is done by they are being chased by someone or they are walking into a social encounter of some kind a. ie: The players are meeting with a shady smuggler trying to lower the price he is going to pay them for a job they are about to do.
If you watch a lot of series TV you will indefinitely notice this happens a lot. Watch some episodes of Star Wars: Rebels. Usually the best of these shows start out with the action already going on. Start off the episode they are being chased by tie fighters or they are in the middle of a job stealing crates from a imperial depot.
You just need to let them know what is going on at the start and boom they are there.