The necessary condition for Retevis H-777, RT21 and RT-5R compatible is same frequency and same CTCSS.
H-777:400-470MHz UHF analog radio
RT21:400-480MHz FRS analog walkie talkies
RT-5R:136-174/400-520MHz dual band keypad programmable hand-held transceiver
Therefore, there is a common frequency range between the three of them. By programming them, they will be changed to same frequency and CTCSS tone,and they will work together.
Frequency and sub-audio of H-777 and RT21
H-777 default frequency list
RT21 default frequency list
In summary, even if the different walkie-talkie in different group, they can communication with each other after programming to same frequency and CTCSS.It is convenient for your business to achieve all calls.
Similarly, interphones other than the H-777, RT21, and RT-5R can communicate with each other as long as they can set the same frequency and sub-audio.