We're so pleased you found some new friends through playing Adopt Me!
You can add these new friends by selecting the "Friends" menu on the sidebar and pressing "Nearby." This will bring up a full list of everyone on the server so you can easily add them. Pressing "Add Friend" will lead to a Roblox pop-up menu to confirm that you would like to add this user to Roblox.
Usernames have been hidden here to protect these users' as to capture this image, a random public server was joined. Usernames would otherwise be on display when you go to add new friends yourself.
Alternatively, you can bring up the Roblox Menu by clicking the Roblox icon in the top left corner of your screen. The menu will automatically place you so you are viewing a list of everyone on the server and again allow you to add friends through this screen.
Once they have accepted your request, you can find them on your Roblox friends list and play with them on any Roblox experience, including Adopt Me!