A good guideline right here is to work with the mom (and ideally, the father) to-be in order to choose who should go to, and who should be left off the list. This is a fragile circumstance and can cause a number of small headaches (even some major ones).
The problem is, merely, that while it would be perfect to welcome everyone who would wish to go to, that's just not useful; either economically, or simply in regards to planning. Ultimately, decisions will have to be made, and if you can deal with the parents-to-be to make these choices, the opportunities of making wise ones will enhance.
When you've determined who to invite - and this procedure can take a few days of thinking and re-thinking - the next step is to send the invites. Ensure that you do this well in advance of the infant shower. There are 2 major reasons for this.
Firstly, you wish to offer your invitees enough lead time to that if they do have something planned on the infant shower date that they can, if they want, move those plans in order to go to. If you do not supply them with enough notification, even if they wish to change their existing plans, they may not be able to.
Secondly, you want to offer individuals adequate time to RSVP (i.e. confirm their participation). Some people are not the most organized individuals worldwide, and as such they might not RSVP promptly. As such, you wish to provide them a little time to get to this on their ever-growing TO-DO list.
Now, there's another issue here that we should go over. Some individuals think, or just assume truly, that if you do not RSVP, that implies you aren't attending. That's really not technically right. RSVP doesn't suggest (even in the French language from where it comes) that someone is going to go to. It just indicates: please return to me on this.