Send a symmetric key generated by one user to other users (more than two) using a server I am developing a software in which I am using AES for encrypting messages shared in a group.  The key is generated by one user, the admin of the group and needs to be shared with other members. How can I share this key with all other members of the group securely?

Are protocols like Needham-Schroeder, Diffie-Hellman applicable in my case? (I don't know much about cryptography)

PS: Can I encrypt the 'group key' generated by admin using AES, and save it on a server? The other users can request the server for the  encrypted key and then decrypt it. The key used for encrypting and decrypting the 'group key' will be pre-shared (embedded in the software).
The best way is to keep things simple. Once the key has been generated, keep it on the server and make sure only authenticated users can request the key from that location. Make all the clients request the key from the server.