In my experience +/-2 levels from the target is ok, more than that is a stretch but even someone 3 or more levels below can contribute effectively if the character is at all optimized.
I tend to recruit in a 4-5 level band (my latest adventure was for 10-13). Then once I find out the levels of the PCs that are playing. I tailor the encounters for that party.
For example if I budget an AL+1 encounter into my adventure and my party is levels (10, 11, 11, 12, 12, 13), I'll add the +1 to each of those levels (11, 12, 12, 13, 13, 14) and then take the xp of a monster of that level (600, 700, 700, 800, 800, 1000). That total (4600) will be my budget for an AL+1 encounter.