How to list all user names on Solaris 10

Very interesting attempt. Actually, process's supplementary groups (defined in /etc/group) are set by setgroups system call. It requires CAP_SETGID privilege or being root.

So you can do like this:

# id
uid=0(root) gid=0(root) groups=0(root)

# gdb -q id
Reading symbols from id...(no debugging symbols found)...done.
(gdb) b getgroups
Breakpoint 1 at 0x401990
(gdb) run
Starting program: /usr/bin/id 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, getgroups () at ../sysdeps/unix/syscall-template.S:81
81  ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) call setgroups(5, {1, 2, 3, 4, 5})
$1 = 0
(gdb) d 1
(gdb) c
Continuing.
uid=0(root) gid=0(root) groups=0(root),1(daemon),2(bin),3(sys),4(adm),5(tty)
[Inferior 1 (process 8059) exited normally]
(gdb) 

The rsync command doesn't have a mechanism for handling this directly, so I would use a different approach. I would scan the source filesystem tree, collecting the usernames (and groups) of all files present there:

# List of usernames owning files under 'src'
find src -printf "%u\n" | sort -u | tee /tmp/src.users

# List of group memberships for files under 'src'
find src -printf "%g\n" | sort -u | tee /tmp/src.groups

# Copy files to target system
scp -p /tmp/src.{users,groups} dest:/tmp/

I would then ensure that all users existed on the target system ready for rsync to use. Run these commands on the target system:

# List "missing" users
getent passwd | cut -d: -f1 | sort -u | comm -13 - /tmp/src.users

# List "missing" groups
getent group | cut -d: -f1 | sort -u | comm -13 - /tmp/src.groups