People join groups because groups satisfy a range of needs.
Conditions which facilitate group formation:
Proximity – Individuals with similar background, interests, attitudes, values and opinions when have repeated interaction with each other, get a chance to know each other and develop a mutual liking. This facilitates group formation.
Similarity – Being exposed to someone over a period of time makes an individually assess similarities. Assessment of similarities paves the way for formation of groups. Explanation given psychologists are that when people meet similar people, they reinforce and validate their opinions and values. They feel that they are right and thus start liking one another.
Common motives and goals – When a number of people have common objectives or goals, they tend to get together and form a group, which may facilitate goal attainment. For example: Commuters in a train may form a group which can work towards facilitating their travel on the train.