Really depends on the individual community. You gotta get to know them and build trust first.

And if they're actually Amish or one of the dozens of other similar groups that constantly get mistaken for Amish because they're all slightly different.

You show up around harvest time looking for work and eventually marry into the community.
It could take years to gain enough trust before anyone offers you their daughter/you can court unmarried women from the community.

But marriage and building a family within these communities is usually the only way to really become part of them.