Add jsmith to the same groups than ajames and tclark, and change the permissions so that the groups can have the access you want.

Code:

usermod -G ajames,tclark jsmith

Will set secondary groups ajames and tclark to jsmith
Assuming that those are the primary groups for them (many distributions make the group the same that the login user)
A caveat is that it will remove from jsmith any other secondary groups that you don't include in the command.

Code:

gpasswd -a ajames jsmith

Will add group ajames to jsmith without removing the previous secondary groups. But you need to include one at a time.

Code:

groups ajames

Will show groups that ajames belongs to

Of course, the right permissions should be set for the groups to be able to access files

Code:

umask 027

Will give read and execute permissions to group, and none to others.

Code:

umask 007

Will give same permissions to group as to owner, and none to others.

The default mask for when you create a new user is found in the file

Code:

/etc/login.defs

Groups can be a powerful tool.

If you just want to give some privileges to jsmith to go around snooping in other users. Take a look at /etc/sudoers

Code:

#> visudo

Will allow to modify /etc/sudoers as root