What is a Sangha? Do you have to be a Buddhist to join a Sangha? What is a Sangha?
I know that a sangha is group of Buddha’s followers!
But I want to have a more thorough understanding of what a sangha is!
Do you have to be a Buddhist to join a Sangha?
According to the Wikipedia article about Sangha,


It's a Pali word meaning meaning "association", "assembly," "company" or "community"
It usually means the monastic community of ordained Buddhist monks or nuns
As a separate category, those who have attained any of the four stages of enlightenment (for example Sotāpanna), are referred to as the ariya-sangha or "noble Sangha".
The word Sangha sometimes also includes laymen and laywomen who are personally dedicated to the discipline of Dharma-Vinaya
Some lay practitioners in the West these days use the word "Sangha" as a collective term for all Buddhists, but the Pali Canon uses the word parisā (Sanskrit, parisad) for the larger Buddhist community — the monks, nuns, lay men, and lay women who have taken the Three Refuges — reserving ‘Sangha’ for a more restricted use.


So according to these conventional (non-inclusive) definitions, only Buddhists (and of those, not all Buddhists) are in the "Sangha".