You mean are clubs able to join mls2? At least in SoCal for this season that’s done. MLS does not take individual teams. Only clubs that are not subject (at least not currently) to pro rel. the announcement is done in march/april because try outs are under way and the mls olders go dark in June so teams need to be organized before then. The strong preference for mls2 is for the club to have mlsn and those exceptions for mls2 are those clubs mlsn is looking at for full promotion down the road.
Teams are near the end of tryouts. If a team was going to move over wholesale to a new club to become the mls2 team that should have been planned months ago (like Albion did with some fc England teams)
If you mean individual players tryouts are nearing the end but if it’s not one of the top teams and unless your kid is for one of the specialized positions (striker gk) you can probably still find space if they are good enough.