As already stated in many situations it will suffice to ping (Alt+G click) the minimap.

Depending on how good your ally is and how often you have played with him I think the following is a good guideline:


ping once - Look here, I want you to see this.
rapidly multiple pings - Immediate threat, react fast!
chat "scan", ping once - I want you to scan here.
chat "100 min" or "100 gas" - Need 100 minerals or 100 gas.
chat "gg?" - I have nothing, do you have anything that can change the game?
chat "i go ..." - I will use ... strategy.
chat "can i fast expand?" - I want to use the natural, can you cover me?
chat "scout" - Send a scouting worker, I need my resources.
chat "push" or "push?" or "push in x" - Asking when to push or indicating you want to push now or soon.
chat "b" or "back" - You will not commit to this fight and want your ally to back off.


There are many other messages, but these are the ones I use most often.

You should rely on common sense and trust your ally. Assume that if you show him something he will react correctly.

How can I grab a teammate's attention?
There are two specific examples in the Android Floating Action Button guidelines that explain how the button can morph into either new buttons, or a toolbar. 


  If the hallmark of the app is adding file types, a floating action
  button can morph into related actions after it is first touched.





  If a floating action button morphs into a toolbar, that toolbar should
  contain related actions. In this example, the button lets the user
  select the media type to add.




After the button has morphed, how does the user revert back to the previous single button mode?

As I understand it, the rest of the UI is not blocked after these buttons have morphed, so it's not clear how the user can hide the new buttons or revert the morphed shape if they decide not to go through with the new action.

Update

This is more commentary than anything else: I just took a look at Drive to find implementation examples and found an Bottom sheet instead of a FAB morph.


