Former mormon here.
The person who performs the wedding will depend on the law (for example in the UK they would need to be an authorised 'Celebrant' - religious - or an authorised 'Registrar' - civil.) The US will have its own set up for who can perform marriages. One important distinction is that in the UK a marriage must be public meaning that the US mormon practise of temple weddings (only 'worthy' church members can attend) as the sole recognition of marriage is not allowed leading to UK members taking the route of an offical legal marriage (normally in an LDS church and done by the Bishop) and then the same day going to the temple for an evening 'Eternal marriage' performed by a temple official.) Members who don't go straight to the temple ar enormally penalised by not then being allowed to eternally marry till 1 year later as determined by the local church leadership.
In the US the practise of recognising the secret temple ceremony as official has led to lots of angst for 'unworthy' or nevermo family members being excluded from this beautiful family moment. Stupid cult.
In your case it looks like they will be having a public marriage (phew) and in this case the positive aspect of mormonism will be on display. In general the members who attend will be lovely and you'll find that they won't have any great hangs ups. They'll integrate easily, shouldn't be too phased by alcohol, won't get rowdy themselves and will generally be lovely (though they may bunch up but that's normal for weddings anyway.) You may get the odd mormon loudly mentioning something but in general mormons find it rude to intrude their beliefs on someone else's public occasion (surprising but true) and so I wouldn't expect you to get any problems from the mixed nature of the event.
If the wedding is at an LDS meetinghouse then there is stricly no smoking in the building or on the grounds, ditto for drinking alcohol and tea and coffee (so if you are having a meal at the church afterwards don't expect to serve booze or coffee). It may be worth pre-warning nevermos so that the day isn't troubled by awkward moments and if they want to they can arrange a quick trip to a local bar. Also depending on the nature of the invites you may get loads of the local members attend (they have no restrictions with intermingling with worldy people at public events - mormon wards also see themselves as a large family so often feel they have an open invite !) If you ar eeating at the church be aware of moochers. After church events some members will feel entitled to walk off with leftovers. Control this heavily. Don't let people take stuff away unless they donated it in the first place (some people want their dishes back quite understandably). If you have cases of food/drinks police them to make sure they don't get acquired ! This will not likely be a problem if you are eating elsewhere.
Warn the speech givers of the churchy nature of a large part of the crowd. Rude jokes etc. will not go down well and will lead to an awkward atmosphere. Light hearted jokes will be taken well but rude ones will not.
It will be unlikley that there will be any overt proselyting.
Congrats.