Splunk doesn't require any domain membership of any kind. Simply setup forwarders on each machine you want splunk to collect events on, and simply forward them all to one central splunk instance.
There is no authentication or authorization required between forwarders and the indexers (receivers).
If you are collecting logs over remote shares, then that's the only time I can think of when domain credentials are needed. And really that's not a splunk thing at all, it's just that a windows service needs to to run as a non-system user in order for it to access remote shares; but that's not the ideal splunk setup. Using individual forwarders is recommended.
Related docs:
http://www.splunk.com/base/Documentation/latest/Admin/Enableforwardingandreceiving
