What's a good method for testing out a new web page for just a small set of users?

Personally, I utilize my database's users table.

#    fname    lname        email                beta_access       username    [...]
1    James    Doe          [email protected]       true              jdoe
2    Sally    Jones        [email protected]           true              sjones
3    Alex     Jennings     [email protected]      false             ajennings

Using the above example, there are three random users. In your code, you can simply have an if statement to represent beta_access = true. If true, show beta code. If false, do not.

A second method, which I have done on larger sites, is to have a unique ID in the beta_access column. For example:

#    [...]   beta_access    [...]
1            14573
2            57232
3            14573
4            [null]

This way, you can have multiple things you'd like tested at once, but not grant access to each for all users with beta access. Live chat might be represented by 14573, whereas a new contact form UI might be represented by 57232. This is a bit overboard for simple applications, but is quite useful for larger sites.