The top teams almost all have a spot open for a new arrival, but to get that spot, your kid has to basically be a starter-level player (i.e. the top 11). They won't put you in roster #18 when there are kids on the second team who probably deserve a shot.
I do agree with your strategy of making friends and finding a mutual club. I would add to that commuting time as teams can practice ALL OVER the area, even teams that are technically in a specific place ('Bethesda', Arlington, etc). Drive times can be horrible at rush hour so you'll need to factor that into you decision process.