As a more detailed reply (no offense Chrono), here you go:
1. Open a server.
2. Press Ctrl + O (or press the Esc key and click Options).
3. Go to the Controls tab.
4. Scroll down the list of controls until you get near the bottom and look for RTB Controls.
5. Double click the one called Server Control and set a keybind to it.
6. Click Done.
7. Have your friend(s) join your server.
8. Press the keybind you just set.
9. In the Admin tab, click once on their name in the list on the right.
10. Click Add.
11. Select whether you want them to have Super Admin or regular Admin, then click Done.
12. When they join your server, they will automatically be made Admin or Super Admin, whichever one you set it to.
To set a welcome message, open Server Control and in the tab that opens by default, at the top, there's a text box entitled Welcome Message. Change that, click Save, and presto. Your players will now see that message everytime they join your server.
Tip: if you want to say the name of the person joining, for instance, "Welcome to my server, Kyzor.", in the Welcome Message box, type "Welcome to my server, %1." That will display their own name when they join.
Another tip: if you want to add some flair to your welcome message, use hex codes. For instance, "
Welcome to my server, Kyzor."
To do that, type the following in the welcome message box:
<color:0000ff>Welcome to my server, <color:00ff00>%1.
Hex codes are "coded" in RGB, or Red, Blue, Green. So, for example, if you wanted red text, you would type <color:ff0000>. The first two zeros are red, the second two are green, and the third two are blue. Play around with them a bit to get a plethora of different colors.
