According to http://dict.tu-chemnitz.de/deutsch-englisch/Eis.html Eis is neuter.
Sie isst Eis . She eats a ice cream.
Just "ice cream", not "an ice cream" since the German doesn't have ein Eis. (and "a ice cream" is never correct.) Otherwise yes.
Sie hat Eis . She has an ice cream.
Sie essen Eis. They eat an ice cream.
Correct, if you leave out "an".
Sie habt Eis . They have an ice cream .
Sie haben Eis. "They have ice cream".
sie (they) verb forms end in -en.
In general, you can't tell the gender of a noun just by looking at it, so it's something you just have to remember.
There are some kinds of nouns where the shape (e.g. an ending or a prefix) usually corresponds with a given gender (e.g. Ge- is often neuter, -e is often feminine etc.) but even those can have exceptions.
You can tell "she" and "they" apart by the verb form (see the other comments on this page).
Ice versus ice cream is just context -- most of the time, they will both simply be Eis in German and context tells you whether it's pure frozen water or something flavoured or dairy-based.
"Sie" as the formal spech always has a big "S". Sie as in "she" has a small "s" (exception: you start your sentence with "Sie", then the S is big.) The rest is context. And you have to look at the verb. There is a difference between Sie sind groß (you are tall!) and Sie ist groß(she's tall)
Sie can be used for both 'she' and 'they' how to differentiate?
By the verb -- "she" forms end in -t, "they" forms in -en:
- sie isst = she is eating
- sie essen = they are eating
sein "to be" is irregular; while sie ist "she is" ends in -t, sie sind "they are" does not end in -en. This one just has to be memorised.