Make sure you connect with the NHREIA, great group, they have monthly meetings in Manchester and a busy FB group with 3500+ members.

They seem to be the only groups that are willing to do this. 

There is a great group of members on here to connect with.

I found and joined those DU groups on FB (but I'm having trouble being able to post there).

I've used the search feature on this forum but didn't have much luck.Are there any US investors, hard money lenders, or groups that invest in flips outside of the United States, specifically in Europe (Netherlands, Spain, etc.)?

Start with a contractor that has a lot of 5 star reviews.You can join some facebook groups and network with investors in your area.

Even if these relationships don't go anywhere, you'll get a lot from just having a couple of calls.Next I would try to find some local investment groups/communities and try to chat with as many investors as you can to get an idea how other's started out in your market.

Enrique,  Welcome to the group!

There is also podcastguests.com as well as several facebook groups where you can find people to get on other podcasts. 

You can also check meetup.com or search Facebook for real estate investment groups, clubs, or meetings in your area.