Even if you have the right username and password for the remote machine when trying to join, its most likely that UAC is denying your accions.
In latest version you are able to join remote computers to domain but you have to disable UAC on the remote machine witch is no good for security reason.
To bypass UAC in Windows 7 i have come to conclusion that if we alter DCOM security a little we well bypass UAC.
Do this:(Information from Microsoft)
To grant Remote Activation permissions to the SMS Admins group
1.From the Start menu, click Run and type Dcomcnfg.exe.
2.In Component Services, click Console root, expand Component Services, expand Computers, and then click My Computer. On the Action menu, click Properties.
3.In the My Computer Properties dialog box, on the COM Security tab, in the Launch and Activation Permissions section, click Edit Limits.
4.In the Launch Permissions dialog box, click Add.
5.In the Select User, Computers, or Groups dialog box, in the Enter the object names to select (examples): box, type SMS Admins and click OK.
6.In the Permissions for SMS Admins section, select the check box to allow Remote Activation.
7.Click OK twice, and then close Computer Management.