You are correct with the translation. It means to decide something together with someone else.
Entscheiden means to decide. The prefix mit means, that you decide something together with someone else.
You could say:
Wir haben bei dem Thema mitentschieden.
This would mean, that you decided something together with other people.
If you also want to say, what you have decided (for example you decided, that you want to buy some apples):
Wir haben mitentschieden, dass wir Äpfel kaufen.
You could also say, with which persons you have decided something.
Wir haben mit Person1, Person2 und Person3 entschieden, dass wir Äpfel kaufen.
In This case mitentscheiden gets sliced into mit and entscheiden.