For pop/rock, likely the direct answer is you want to sing over a scratch rhythm track. That can be anything from the built-in rhythm pattern on a keyboard on up to having a drummer, and possibly bassist, record a basic track for the entire song. Starting with prepackaged drum loops or programming something simple into a drum machine program is likely a good way to go.
At some level, a metronome accomplishes the same purpose, but the drum track will give more of a feel and provides a lot more support for most singers. The tricky bit is some songs may require a fair bit of sophistication in crafting a good scratch track. Finding an existing song that has the right rhythmic feel and then making a minimal rhythm track that is similar can be useful. One possibility may be to get such a track from the folks you are working with. If there is little notion of what that might sound like, then figuring that out might be a good first step.
This requires being setup to record while listening to playback. Ideally this would be actual synced multitrack recording so the end result is a vocal track that can be played back with the rhythm track, but the important part is that the vocal sits on top of some rhythm track, not the specific one. Ending up with just a vocal would be fine if it has adequately defined rhythm.
This applies to fragments of a song as well as an entire piece. It is very useful if recording sections to see how they fit together as they'll have more consistent time.