Sending the command(s) spawned by xargs to background

Use the --max-procs / -P option to run xargs targets in the background. From the man page of GNU xargs version 4.2.27:

--max-procs=max-procs, -P max-procs

Run up to max-procs processes at a time; the default is 1. If max-procs is 0, xargs will run as many processes as possible at a time. Use the -n option with -P; otherwise chances are that only one exec will be done.

(You may want to combine this with -n 1 to makes sure that there is a new process for each file you want to compress)

You could probably make a very quick shellscript to call compress.

#!/bin/sh 
# call it 'compbg' and chmod a+x
compress $* &

then

find . -type f  -mtime +7 | tee compressedP.list | xargs -I{} compbg {}

Although I think you might be happier using this xargs argument:

 -P maxprocs
         Parallel mode: run at most maxprocs invocations of utility at once.

This command should find / tee / compress 10 files at a time until its done, as well as returning control immediately to the calling script/shell.

find . -type f  -mtime +7 | tee compressedP.list | xargs -I{} -P10 compress {} &

For SunOS you may have a look at GNU Parallel http://www.gnu.org/software/parallel/

find . -type f  -mtime +7 | tee compressedP.list | parallel compress

It has the added benefit of not terminating incorrectly if the filename contains ' " or space. Adding -j+0 will make it run one compress per CPU core.