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You certainly need to have a letter of invitation/sponsorship from an employer if you want to apply for a work visa, which includes non paid (i.e., volunteer work), but I have never heard of anyone needing an invitation for a Tourist Visa, and in fact it is generally very easy to obtain a 'Visa on Arrival'.
Who told you that you need an invitation, and what is your nationality? (Pakistan nationals may have problems obtaining a Bangladesh Visa).

Thank you for your reply.

I have a Canadian Passport so I went to this website for Canadians: 

It is the High Commission of Bangladesh in Ottawa-Canada.

"3. State clearly the purpose of your visit to Bangladesh and attach documents in support of your visit. Please enclose a letter from the person / business office / government or non-government organizations / other office in Bangladesh who has extended invitation to you."

But, after reading your other post about Dhaka -in which I do not care spending time- having the people who do not care about each other I will not go there to stay more than a few days!  I knew everything about Bengalis but this very hard to accept for me.

You have been more helpful than you think.