16 That ye submit yourselves unto such, and to every one that helpeth with us, and laboureth.
[AD 407] John Chrysostom on 1 Corinthians 16:16
"That ye also be in subjection unto such, that is, "that ye take a share with them both in expenditure of money, and in personal service: that ye be partakers with them." For both to them the labor will be light when they have comrades, and the results of their active benevolence will extend to more.

And he said not merely, "be fellow-helpers," but added, "whatsoever directions they give, obey;" implying the strictest obedience. And that he might not appear to be favoring them, he adds, "and to every one that helpeth in the work and laboreth." "Let this," saith he, "be a general rule: for I do not speak about them individually, but if there be any one like them, let him also have the same advantages." And therefore when he begins to commend, he calls upon themselves as witnesses, saying, "I beseech you, ye know the house of Stephanas." "For ye also yourselves are aware," saith he, "how they labor, and have no need to learn from us."

[AD 418] Pelagius on 1 Corinthians 16:16
Paul says this, because the people he is referring to dwell among the Corinthians, who can profit greatly from their presence.