Sum of values based on usergroupid, using two tables

[brownafroduck]

I'm a webmaster relatively inexperienced with databases and mySQL, so I am looking for any sort of pointer in the right direction like this. I've attempted to write a few SUM statements (is that even what you call them? statements?) using resources around the web, but I just can't seem to get this right.

I have two tables user and userfield.

In the user table I have the userid and usergroupid values.
In the userfield table I have the userid and userfield16 (an integer value representing points) values.

I would like to total the userfield16 points based on the usergroupid and have this value displayed on the forumhome index.php.

Make sense?