In the singular, it is quite clear that one uses "I" when referring to a third party and oneself, as in:

Bob and I are going to build an aircraft.

However, in the plural, it is a lot less clear. For example, if a group of boy scouts are talking about their next project and want to include an outsider, e.g. Sarah:

Sarah and we are going to build an aircraft.

Or would it be:

Sarah and us are going to build an aircraft.

Or even:

Sarah and ourselves are going to build an aircraft.

Please help as otherwise I'll have to shelve the whole aircraft project.