Hi Alon,
There are two options:
- Create a new user account (or use an existing Google account that is not associated with any MCC or AdWords account) and grant it access to the AdWords account.
- Use a different (or new) MCC and grant it access to the AdWords account.
The advantage of approach #2 is that they could reuse the MCC for other accounts. With approach #1, the (non-MCC) user account will only be able to access the one AdWords account.
The reason for this is that each Google account can be directly associated with at most one AdWords account or at most one MCC account because AdWords needs to know where to take the user when they log in to AdWords. If the user is associated with an MCC, he'll be taken to My Client Center, where he can then add more accounts to the MCC. If the user is associated with an AdWords account, he'll be taken to the AdWords UI but won't be able to add more accounts.
Cheers,
Josh, AdWords API Team