Within primary groups each member will tend to know all of the other members and will interact on a face to face basis. An example of this type of group would be a person's immediate family or their work group. Secondary groups are larger than basic primary groups, and while they also share common values and interact, this is not done to the same extent as with primary groups. Examples of secondary groups may include department stores or factories, where the primary groups are individual departments or sections. Both primary and secondary groups can be perceived as formal or informal. It is stated by Mullins (2002) that "Formal groups are created to achieve specific organisational objectives and are concerned with the co-ordination of work activities" and that "Informal groups are based more on personal relationships and agreement of group members than on define role relationships. They serve to satisfy psychological and social needs not related necessarily to the tasks to be undertaken". Reference groups are groups which a person may identify with or aspire to, but they may never meet that group. This type of group can influence people who may not in any clear sense be a member of that group. Reference groups can also be either ascribed, of which membership is automatic, or acquired, where membership is voluntary. A board of directors within a company is an example of a reference group to an employee who wishes to advance their career within that company. When referring to the size of group, it is generally thought that if it exceeds a certain number of members its dynamic forces will lose much of their intensity. Depending on the context of the group that number may vary, but the usual dividing line between small and large groups is between fifteen and twenty. .
Four main phases of development of groups have been identified by Tucker (1965). These are forming, storming, norming and performing.