Roughly: I want to give a party and surely everybody would like to sit down there from time to time. The person is most probably trying to get some chairs for her party. nämlich = is difficult to translate. It is not necessary, to use this word in the sentence, Ich möchte eine Party machen (geben would be better German) would be enough, nämlich is stressing that she mentioning it, because it is part of the explanation. Maybe like "you see" in English as in "I want to give a party, you see, ..." sicher = surely, certainly mal sitzen means, that the people would like to sit down from time to time or at least once.

"nämlich" indicates that this sentence gives the reason for something said in the sentence before. "Ich gehe in die Stadt. Ich mõchte nämlich ein Buch kaufen."