If it is possible to match you to an open group when entering a system for a quest the game will automaticaly do so. This can be both a blessing and curse because while yes its an MMO and playing with others is fun, you will be forced to share your loot your anomalies. As well as that, for each person with you the enemies become stronger. With random people added that are not coordinated this can overwelm you quickly. You can disable this by entering your social menu and turning group style to closed. This alows you to invite your friends and recieve invites from others but not be matched to random groups upon entering systems.
However, you can organize your own team by inviting your friends to your team for a little PVP or whatever fits your liking.