tacoma

A month into your relationship?

Had you had any discussion about exclusivity?

Did she state she was going to see you and only you?

I just know I never went exclusive so quickly so unless she had specifically told me we were an exclusive "couple" I`d let it go.

Edit

I might let it go anyway, after only a month knowing someone who really knows what they want with them?

The fact that she wants to invite him to your wedding is actually more troubling to me.
Are they still friends?
Does she see him regularly?
You need to learn about boundaries in a committed relationship before tying the knot because being friends with an ex is unacceptable.