Is it proper to go to someone's house uninvited we have neighbors who have backyard pool parties often and we are never invited beforehand and my husband constantly just pops up there?

I suspect that you remain behind because you realize your neighbors have not extended an invitation. Please explain to your husband that while it may seem a casual backyard event it is inappropriate to impose on your neighbors privacy. Maybe your husband desires to socialize with them; he should host his own backyard party and extend an invitation to them. Of course with the understanding that the gesture may not be reciprocation nor should it be expected.