Every user on the Force.com platform requires a primary user license and different feature licenses. We can add multiple feature licenses to a user but can add only one user license.
Let's find out how many licenses are available in your organization:
Go to Setup | Administration Setup | Company Profile | Company Information.
Below the Company Profile in the related list, the number of licenses available for use is shown in the following screenshot:
To apply the user licenses, perform the following steps:
Go to Setup | Administration Setup | Manage Users | Users.
Create New User or Edit an existing user.
Tip
A user once created cannot be deleted. We can only make a user active or inactive. Creating an active user consumes a user license, which is directly linked to the billing of Salesforce usage. It is recommended that you practice this in a developer organization. Consult your administrator if you are creating a new user in the production organization.
Select the user license...