The main problem with that solution is that it serves two very different needs. If it's a plain link, as for example "http://foo.bar/joingroup_x", there isn't much you can do. Either you rename the last part of the url so it's clear, that by hitting that link you're going to join a group or install the app (like "http://foo.bar/install_app_and_join_group") or you place a description somewhere near that link which must be styled in a way that the user would notice it.
Another way could be to use a button with a descriptive label on it.
Both not nice solutions though but you need to somehow to communicate the two different intents.
I don't know your motivation to implement it that way, but I think you wanted to lower the effort needed to join the App. You may consider to split those two functions again, as it seems that the built-in install function isn't really needed and maybe don't really fullfill its function.