With no code posted I can not give you specifics but... Here goes...
In the addin there must be some code to create the toolbar. In the
ThisWorkbook object add in something like this..
Private Sub Workbook_AddinInstall()
When the addin is installed the toolbar will then be created on the users
machine. As to the second question, just save a copy of the .xla file on a
network drive somewhere that everyone can access. Have them try to install an
addin and select browse to find the xla file. They will be prompted to save a
local copy of the xla that they will have to do. If they don't you will get
shaing violations. To find the xla on your system, look in
C:\Documents and Settings\Your Name\application data\microsoft\addins