How can I grab my local changelist and send it to someone else in Perforce?

set P4DIFF=C:\cygwin\bin\diff.exe
p4 diff -du -c 12345 > patch-to-head.diff

# On Other machine
patch -p1 < patch-to-head.diff

I may be wrong on the env var there, and you might have to do some fixups on the diff file, but the general idea is that you generate a GNU Unified Diff, that you can send to people that can use GNU patch to apply it.

P4 now has native support for the shelve operation in 2009.2. Both the client and server need to be at least 2009.2 to use this.

http://blog.perforce.com/blog/?p=1872

To shelve your pending changelist YOURCHANGE:

p4 shelve [-c YOURCHANGE]

The other user then does:

p4 unshelve -s YOURCHANGE [-c THEIRCHANGE]

In either case the "-c" arg can be omitted to use the default changelist instead. In the p4 shelve case it means a new changelist will be created and the files in your default changelist will be shelved into it; in the p4 unshelve case it means the shelved files from your changelist will be unshelved into the other user's default changelist.