It depends what you mean by 'a couple'. If by 'a couple' you mean a group of two people or things, then you should say 'is'. In this case, 'couple' is the subject. You should be aware that that when you use 'couple' to refer to a group of two people, the implication is that they're lovers. This is what I'd use you meant without context. If by 'a couple' you mean 'a few', then say 'are'. In this case the subject is omitted. Ex. "I see a lot of birds. A couple (of them) are..."

A couple are supposed to be... - this is the correct one. If you want to talk about couples in general this is a more natural way of saying it: "Couples are supposed to be..."

In British English, we are completely flexible about the issue of singular and plural when it comes to singular words which describe a group (or in this case, a pair) of people. If we are considering them as individuals, then we can use a plural verb and plural pronouns (they,their, them), or if it seems more appropriate to the context, we can also treat 'couple' as a singular noun. In this case, I would say 'are supposed to be'. American English tends to be more rigid in this regard. Nouns such as family, team and so on are always singular, and I suspect that couple would be too. Let's see if any American English speakers can offer any insights on this question.

Thank you very much!

I see you already have the question answered so I thought that I would add something to finish off your sentence. ''A couple of my friend are supposed to be coming to the party'' Also, ''Have you seen David and Debbie? ANSWER ''No, I don't know who they are, but there is a couple sat over there in the corner, perhaps that might be them'' A couple = two people going out together.