Also you can add users to different groups into the site or also you can remove users from the SharePoint groups. To add users go to Site Actions ->Sit Permissions as shown in the figure below. 
This will open the Permissions page as shown in the figure below.
Here you can see groups like Members, Owners, Visitors, Viewers etc.
In the ribbon there are options to give permissions to users, also options to create group etc. Also you can add users to existing groups.
To grant permissions, click on the Grant Permissions link from the Ribbon which will open the Grant Permission dialog as shown in the figure below:
Here in the Select Users add the user and in the Grant Permissions section you can add the above user to any existing user group by selecting the first radio button or you can you can also grant permission directly by choosing the 2nd radio button. And in the Send E-Mail section, If you want to send a wel come main then you can check the Send Welcome mail check box and also you can add your personal message. Then click on OK.
If you want to add users to existing permissions group then click on the group on which you want to add user as shown in the figure below.
This will open the user group page. There to add a new user click on the New -> Add Users as shown in the figure below.
This will open of the Grant Permissions dialog as show in the figure below:
In this dialog add the user name and you can add any number of users and Send E-Mail section you can select the check box to send the email to user and also you can write the personal message also.