At first I thought that I was missing out on having fellow founders to rely on but from a purely performance related POV, by demo day the single founders in the group (about 4/15 companies) were better prepared, further along and were making more income than those with team members.
In fact the more team members there were, the worse they seemed to do. 2 teams didn't even make it to demo day due to falling outs between the founders.
Decisions, paperwork and simple tasks that single founders flew through were serious points of contention with the bigger teams (4+)
I think co-founders for the sake of it is a mistake.
Sure, having a great team is always more productive than being a single founder. However a bad team is not even worth risking.
I'd take up their invitation to their events as that's a generous offer. They obviously saw potential in your product. Don't let them pressure you into building a team if you're happy with how it's going.