What year did Dominica join the commonwealth and why?

Dominica is in the Commonwealth of Nations, but is not a Commonwealth Dominion (now referred to as "Realm"). Meaning, essentially, that, while they do acknowledge the British monarch as head of the Commonwealth, the British monarch retains no title pertaining to Dominica (whereas Commonwealth Dominions accept the British monarch as head of state).
Dominica joined the Commonwealth of Nations in 1978. While no specific reason is given, it was likely for the incentives offered by the trade agreement which exists between members of the Commonwealth of Nations. Additionally, most countries with ties to British colonialism have opted to remain in the Commonwealth of Nations (with some exceptions, such as the United States, the Republic of Ireland, and Zimbabwe, which was a member of the Commonwealth, but voluntarily withdrew after being suspended).
Dominica is in the Commonwealth of Nations, but is not a Commonwealth Dominion (now referred to as "Realm"). Meaning, essentially, that, while they do acknowledge the British monarch as head of the Commonwealth, the British monarch retains no title pertaining to Dominica (whereas Commonwealth Dominions accept the British monarch as head of state).
Dominica joined the Commonwealth of Nations in 1978. While no specific reason is given, it was likely for the incentives offered by the trade agreement which exists between members of the Commonwealth of Nations. Additionally, most countries with ties to British colonialism have opted to remain in the Commonwealth of Nations (with some exceptions, such as the United States, the Republic of Ireland, and Zimbabwe, which was a member of the Commonwealth, but voluntarily withdrew after being suspended).