Entryism involves the members of the small group joining a larger one with the aim of building influence or gaining recruits. This was done by the Militant Tendency in Britain who joined the Labour Party and managed to get a controlling influence in the Young Socialists and Liverpool Council. Many other Trotskyist groups have attempted similar feats but failed to gain the influence of the Militant.
An important thing to note is that entryism does not involve dissolving the small organisation into the larger one. Entryism is generally (but not always) done secretly and often organisations run on democratic centralist lines, but is also found in the far-right entering mainstream right-wing groups, e.g. British National Party members joining the UK Independence Party.
See also: infiltration.