how to other users allowed to use my server?

Make sure allow remote logins is set to ON and that you are not blocking port 1433 with any desktop firewalls.

Add their user accounts in the security section within Management studio.

Otherwise, there might be something preventing them from attaching to it. If you have already added their accounts, then you may be receiving an error message. Post the error message and we can provide more help about that.

you may want to check this out: How to: Create a SQL Server Login If users are part of the domain you can add their AD account as @CirqueDeSQLeil suggested, otherwise create a sql login and they will be able to connect to the instance using the login and password.

The other answers on here are all excellent, but they are all assuming that your other users are trying to connect in some fairly low-level way such as through SSMS.

If your users are not themselves database programmers then a better answer may be to create an give them access to an application that permits them to interact in a more high-level abstracted way. Sometimes, the best way to let someone access your database is indirectly such as through a web application, or a more traditional desktop application, or even through Access application under the right circumstances.

Are your users separated from SQL Server by a firewall?

Lacking details in the question leaves us throwing ideas out and is a little bit of a waste of everyone's time; writers and readers. But, we are here to help! And if an educated guess gets more information then we'll do it. Besides... others who come looking may benefit :)

@kavita - Please try to put more details into your question so we can focus our efforts and get you a solution.