The best approach is the old style one of starting new players at 1st, but adventure separate from high level PCs until they've levelled up. Second best is to start them higher, 3rd or 4th is no problem but starting new PCs at 10th or 12th has not worked well for me. I'd say don't have new 1st levellers adventuring with existing 5th+ PCs.
In my current PBEMs, I do:
1. Moldvay B/X D&D: All new PCs start with 5,000 XP, ca 3rd level, adventure together.
Advancement is pretty slow, we've played current campaign a year or so, one PC just reached 5th, one who came in from a prior campaign at 5th just made 6th.
2. C&C Wilderlands: All PCs start at 1st, not planning to accept new PCs once group reaches ca 4th-5th level unless I want to have 2 adventuring parties.