Okay, so this is simple.

1. populate the inventory file with hosts and the username to add like this:

myhost username=myname
theirhost username=theirname
theirhost username=myname

2. Add any ssh keys required to the keys directory, one key per file, as myname.pub.

2a. If you want to add a key to all users, stick it in the keys/global/ directory.

3. make

3a. make can take LIMIT= or INVENTORY= arguments