Hi,
When you installed K2, you should have specified the account to be used as the K2 service account. You should be using that account. But I have no idea what you are trying to do here. It will help if you can provide us with more details what you are trying to do? Since you mentioned SharePoint, are you working on integrating K2 with SharePoint?
@avril20 For the sake of simplicity I would recommend use K2 service accont which resides in the same domain as your K2 server, just because I saw some minor issues with K2 setup manager with creating SPNs/configuring Kerberos when K2 service accoun resides in another domain.
As for "If we can only use one service account, we are limited to only those users having access." I'm not sure I'm following/understand your problem, can you provide more details? From K2 POV you should just configure multiple domains via Workspace + Workspace web.config + AD Servrice2 SI and you are good to go.