Your friends can start off buying Investor Shares in the fund(s) they like. Once they hit the Admiral Shares minimum, they can convert over to them[0].

Many of their funds also have corresponding ETFs, and I've found that most (all) have similar or lower expense ratios, and the minimum buy is a single share (of course, you can't buy fractional shares like you can with mutual funds, so that can be annoying).

[0] https://investor.vanguard.com/mutual-funds/convert-to-admira...