Yes, you can. There are two things that might be considered deactivating a user:

1. There are grey and green dots toward the right end of a user's entry on the USERS page:

If you have clicked one and made it grey that person's account is deactivated. All you need to do is click on the grey dot to reactivate them. It will prompt you to type "ACTIVATE" (all caps) and then click the YES ACTIVATE IT! button.

- OR -

If you have archived them, you can "unarchive" them as follows:

Click on SHOW ARCHIVED above the list of users

Find the unarchive icon and click it. Type "UNARCHIVE" (all caps) and click the YES UNARCHIVE IT! button.

The user is now accessible again.