/use [@mouseover,help] Power Infusion
Mouseover is easiest so you don’t have to manually assign names. Hover the raid/party frame and hit it.
Alternatively you can set whoever you want as Focus and
/use [@focus] Power Infusion
But I would use hover as it is more flexible and once you start you won’t be able to stop. Just keep the general function bound to a key and don’t make a bespoke macro for a single entity.
To make it a button you can use on hover for someone else or if solo just hit the button no hover for you
/use [@mouseover,help][]Power Infusion
If you hover a friendly, PI on them. If no over and you’re by yourself just hit the button and it will cast on you
The syntax [@whatever] followed by empty [] allows you to do both without needing to type [@whatever,help]Spell; Spell
This is useful for Silence, too.
/use [@mouseover,harm][]Silence
Include harm,help otherwise it will try casting no matter what you are over and nameplates can be funky
This allows you to silence another mob/player by nameplate hover without having to target, silence, retarget - or silence your current target when not hovering. Use this all the time in PvP and on my alt tank to interrupt, grab escaping aggro, etc without dropping current target. Sometimes you want to keep your target.
What really helps visually is using WeakAuras and placing a small green circle anchored to my mouse pointer so I don’t lose it. It’s always visually present and easy to see with a screen full of particle effects and nameplates. Set to max layer so it’s on top of everything.